Create setup file InstallData.php

File: app/code/Magemonkeys/Customer/Setup/InstallData.php

<?php

namespace MagemonkeysCustomerSetup;

use MagentoEavSetupEavSetup;
use MagentoEavSetupEavSetupFactory;
use MagentoFrameworkSetupInstallDataInterface;
use MagentoFrameworkSetupModuleContextInterface;
use MagentoFrameworkSetupModuleDataSetupInterface;
use MagentoEavModelConfig;
use MagentoCustomerModelCustomer;

class InstallData implements InstallDataInterface
{
	private $eavSetupFactory;

	public function __construct(EavSetupFactory $eavSetupFactory, Config $eavConfig)
	{
		$this->eavSetupFactory = $eavSetupFactory;
		$this->eavConfig       = $eavConfig;
	}

	public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
	{
		$eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);
		$eavSetup->addAttribute(
			MagentoCustomerModelCustomer::ENTITY,
			'sample_attribute',
			[
				'type'         => 'varchar',
				'label'        => 'Sample Attribute',
				'input'        => 'text',
				'required'     => false,
				'visible'      => true,
				'user_defined' => true,
				'position'     => 999,
				'system'       => 0,
			]
		);
		$sampleAttribute = $this->eavConfig->getAttribute(Customer::ENTITY, 'sample_attribute');

		// more used_in_forms ['adminhtml_checkout','adminhtml_customer','adminhtml_customer_address','customer_account_edit','customer_address_edit','customer_register_address']
		$sampleAttribute->setData(
			'used_in_forms',
			['adminhtml_customer']

		);
		$sampleAttribute->save();
	}
}

Run command line to install the module: php bin/magento setup:upgrade and php bin/magento setup:static-content:deploy

Then check the result. It will show like this: