Magento 2 create a custom attribute and save data

Create a custom module and follow the below step

Create InstallData.php file in VenderModulenameSetup

<?php

namespace VenderModulenameSetup;

use MagentoFrameworkModuleSetupMigration;
use MagentoFrameworkSetupInstallDataInterface;
use MagentoFrameworkSetupModuleContextInterface;
use MagentoFrameworkSetupModuleDataSetupInterface;

class InstallData implements InstallDataInterface {

    private $customerSetupFactory;

    public function __construct(MagentoCustomerSetupCustomerSetupFactory $customerSetupFactory) {
        $this->customerSetupFactory = $customerSetupFactory;
    }

    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context) {
        $installer = $setup;
        $installer->startSetup();
        $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);
        $entityTypeId = $customerSetup->getEntityTypeId(MagentoCustomerModelCustomer::ENTITY);

        $customerSetup->addAttribute(MagentoCustomerModelCustomer::ENTITY, "custom_attribute", array(
            "type" => "int",
            "backend" => "",
            "label" => "Custom Attribute",
            "input" => "select",
            "visible" => true,
            'source' => 'MagentoEavModelEntityAttributeSourceTable',
            "required" => false,
            "default" => "",
            "frontend" => "",
            "unique" => true,
            "note" => "",
        ));

        $employee = $customerSetup->getAttribute(MagentoCustomerModelCustomer::ENTITY, "custom_attribute");

        $employee = $customerSetup->getEavConfig()->getAttribute(MagentoCustomerModelCustomer::ENTITY, 'custom_attribute');
        $used_in_forms[] = "adminhtml_customer";
        $used_in_forms[] = "checkout_register";
        $used_in_forms[] = "customer_account_create";
        $used_in_forms[] = "customer_account_edit";
        $used_in_forms[] = "adminhtml_checkout";
        $employee->setData("used_in_forms", $used_in_forms)
                ->setData("is_used_for_customer_segment", true)
                ->setData("is_system", 0)
                ->setData("is_user_defined", 1)
                ->setData("is_visible", 1)
                ->setData("is_used_in_grid", 1)
                ->setData("is_visible_in_grid", 1)
                ->setData("is_filterable_in_grid", 1)
                ->setData("is_searchable_in_grid", 1)
                ->setData("sort_order", 100);
        $employee->save();

        $installer->endSetup();
    }

}