If you want to add custom column in product grid, then follow below step to create custom module to add custom column in grid.

Step 1) app/code/Magemonkeys/SalesordercreategridCustomattr/registration.php

<?php
MagentoFrameworkComponentComponentRegistrar::register(
    MagentoFrameworkComponentComponentRegistrar::MODULE,
    'Magemonkeys_SalesordercreategridCustomattr',
    __DIR__
);

Step 2) app/code/Magemonkeys/SalesordercreategridCustomattr/etc/module.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Magemonkeys_SalesordercreategridCustomattr" setup_version="1.0.0">
    </module>
</config>

Step 3) app/code/Magemonkeys/SalesordercreategridCustomattr/etc/di.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="MagentoSalesBlockAdminhtmlOrderCreateSearchGrid" type="MagemonkeysSalesordercreategridCustomattrBlockAdminhtmlOrderCreateSearchGrid" />
</config>

Step 4) app/code/Magemonkeys/SalesordercreategridCustomattr/Block/Adminhtml/Order/Create/Search/Grid.php

<?php
namespace MagemonkeysSalesordercreategridCustomattrBlockAdminhtmlOrderCreateSearch;

class Grid extends MagentoSalesBlockAdminhtmlOrderCreateSearchGrid
{
    /**
     * @param MagentoBackendBlockTemplateContext $context
     * @param MagentoBackendHelperData $backendHelper
     * @param MagentoCatalogModelProductFactory $productFactory
     * @param MagentoCatalogModelConfig $catalogConfig
     * @param MagentoBackendModelSessionQuote $sessionQuote
     * @param MagentoSalesModelConfig $salesConfig
     * @param array $data
     */
    public function __construct(
        MagentoBackendBlockTemplateContext $context,
        MagentoBackendHelperData $backendHelper,
        MagentoCatalogModelProductFactory $productFactory,
        MagentoCatalogModelConfig $catalogConfig,
        MagentoBackendModelSessionQuote $sessionQuote,
        MagentoSalesModelConfig $salesConfig,
        array $data = []
    )
    {
        $this->_productFactory = $productFactory;
        $this->_catalogConfig = $catalogConfig;
        $this->_sessionQuote = $sessionQuote;
        $this->_salesConfig = $salesConfig;
        parent::__construct($context, $backendHelper, $productFactory, $catalogConfig, $sessionQuote, $salesConfig, $data);
    }

    /**
     * Prepare collection to be displayed in the grid
     *
     * @return $this
     */
    protected function _prepareCollection()
    {
        $attributes = $this->_catalogConfig->getProductAttributes();
        /* @var $collection MagentoCatalogModelResourceModelProductCollection */
        $collection = $this->_productFactory->create()->getCollection();
        $collection->setStore(
            $this->getStore()
        )->addAttributeToSelect(
            $attributes
        )->addAttributeToSelect(
            'sku'
        )->addStoreFilter()->addAttributeToFilter(
            'type_id',
            $this->_salesConfig->getAvailableProductTypes()
        )->addAttributeToSelect(
            'gift_message_available'
        );

        $collection->joinField(
            'qty_in_stock',
            'cataloginventory_stock_item',
            'qty',
            'product_id=entity_id',
            '{{table}}.stock_id=1 AND {{table}}.website_id=0',
            'left'
        );

        $this->setCollection($collection);

        $parent = get_parent_class($this);
        $parentclass = get_parent_class($parent);
        return $parentclass::_prepareCollection();
    }

    /**
     * Prepare columns
     *
     * @return $this
     */
    protected function _prepareColumns()
    {
        $this->addColumn(
            'entity_id',
            [
                'header' => __('ID'),
                'sortable' => true,
                'header_css_class' => 'col-id',
                'column_css_class' => 'col-id',
                'index' => 'entity_id'
            ]
        );
        $this->addColumn(
            'name',
            [
                'header' => __('Product'),
                'renderer' => MagentoSalesBlockAdminhtmlOrderCreateSearchGridRendererProduct::class,
                'index' => 'name'
            ]
        );
        $this->addColumn('sku', ['header' => __('SKU'), 'index' => 'sku']);
        $this->addColumn('customsku', ['header' => __('Custom SKU'), 'index' => 'customsku']);
        $this->addColumn(
            'price',
            [
                'header' => __('Price'),
                'column_css_class' => 'price',
                'type' => 'currency',
                'currency_code' => $this->getStore()->getCurrentCurrencyCode(),
                'rate' => $this->getStore()->getBaseCurrency()->getRate($this->getStore()->getCurrentCurrencyCode()),
                'index' => 'price',
                'renderer' => MagentoSalesBlockAdminhtmlOrderCreateSearchGridRendererPrice::class
            ]
        );

        $this->addColumn(
            'in_products',
            [
                'header' => __('Select'),
                'type' => 'checkbox',
                'name' => 'in_products',
                'values' => $this->_getSelectedProducts(),
                'index' => 'entity_id',
                'sortable' => false,
                'header_css_class' => 'col-select',
                'column_css_class' => 'col-select'
            ]
        );

        $this->addColumn(
            'qty',
            [
                'filter' => false,
                'sortable' => false,
                'header' => __('Quantity'),
                'renderer' => MagentoSalesBlockAdminhtmlOrderCreateSearchGridRendererQty::class,
                'name' => 'qty',
                'inline_css' => 'qty',
                'type' => 'input',
                'validate_class' => 'validate-number',
                'index' => 'qty'
            ]
        );

        $this->addColumn(
            'qty_in_stock',
            [
                'header' => __('Stock'),
                'type' => 'number',
                'index' => 'qty_in_stock'
            ]
        );

        $parent = get_parent_class($this);
        $parentclass = get_parent_class($parent);
        return $parentclass::_prepareColumns();
    }
}

Step 5 : After place above code, please run below mentioned commands

- php bin/magento setup:upgrade
- php bin/magento setup:static-content:deploy
- php bin/magento cache:clean

That’s it…

Now refresh your sales order, create grid page and see that your custom column will be reflecting there.