Create a custom filed on the customers section

Create setup file InstallData.php

Firstly, we will create the InstallData.

File: app/code/<vendorname>/<foldername>/Setup/InstallData.php

<?php
namespace Mageplaza\HelloWorld\Setup;

use Magento\Eav\Setup\EavSetup;
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;

class InstallData implements InstallDataInterface
{
	private $eavSetupFactory;

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

}

Define the install() method

After that, we have to define the install() method and create eav setup model:

public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
	{
		$eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);
	}
	Next, we will use eavSetup object to add attribute:
public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
	{
		$eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);
		$eavSetup->addAttribute(
			\Magento\Customer\Model\Customer::ENTITY,
			'sample_attribute',
			[
				'type'         => 'varchar',
				'label'        => 'Sample Attribute',
				'input'        => 'text',
				'required'     => false,
				'visible'      => true,
				'user_defined' => true,
				'position'     => 999,
				'system'       => 0,
			]
		);
}

Create custom attribute

Finally, we need to set the forms in which the attributes will be used. In this step, we need define the eavConfig object which allow us to call the attribute back and set the data for it. And the full code to create customer attribute is:

File: app/code/Mageplaza/HelloWorld/Setup/InstallData.php

<?php

namespace Mageplaza\HelloWorld\Setup;

use Magento\Eav\Setup\EavSetup;
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;
use Magento\Eav\Model\Config;
use Magento\Customer\Model\Customer;

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(
			\Magento\Customer\Model\Customer::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();
	}
}