Magento 2: Create custom Customer Attribute (Using Patch Data for Magento version >= 2.3.x )

addCustomCustomerAttribute.php

<?php 
/**
 * @license http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 * @author Phuong LE <[email protected]>
 * @copyright Copyright (c) 2020 Men In Code Ltd (https://www.menincode.com)
 */

namespace MyNamespace\MyModule\Setup\Patch\Data;

use Magento\Customer\Model\Customer;
use Magento\Customer\Setup\CustomerSetupFactory;
use Magento\Eav\Model\Entity\Attribute\SetFactory as AttributeSetFactory;
use Magento\Framework\Setup\Patch\DataPatchInterface;

class AddCustomCustomerAttribute implements DataPatchInterface 
{
    /**
     * @var CustomerSetupFactory
     */
    private $customerSetupFactory;
    /**
     * @var AttributeSetFactory
     */
    private $attributeSetFactory;


    /**
     * AddCustomCustomerAttribute constructor.
     * @param CustomerSetupFactory $customerSetupFactory
     * @param AttributeSetFactory $attributeSetFactory
     */
    public function __construct(
        CustomerSetupFactory $customerSetupFactory,
        AttributeSetFactory $attributeSetFactory
    ) {
        $this->customerSetupFactory = $customerSetupFactory;
        $this->attributeSetFactory = $attributeSetFactory;
    }
  
    /**
     * {@inheritDoc}
     */
    public static function getDependencies()
    {
        return [];
    }

    /**
     * @inheritDoc
     */
    public function getAliases()
    {
        return [];
    }
  
    /**
     * {@inheritDoc}
     */
    public function apply()
    {
        /** @var \Magento\Customer\Setup\CustomerSetup $customerSetup */
        $customerSetup = $this->customerSetupFactory->create();
        $customerSetup->removeAttribute(Customer::ENTITY, 'my_custom_attribute');
        $customerSetup->addAttribute(
            Customer::ENTITY,
            'my_custom_attribute',
            [
                'group' => 'General',
                'type' => 'text',
                'label' => 'My Label',
                'input' => 'text',
                'position' => 160, // position defined in the admin customer form
                'user_defined' => true,
                'required' => false,
                'system' => false, // Allow to save values , @see Magento\Customer\Model\Metadata\CustomerMetadata::getCustomAttributesMetadata()
            ],
            /** Boolean example
            [
                'type'         => 'int',
                'label'        => 'Boolean Attribute',
                'input'        => 'boolean',
                'backend'      => \Magento\Customer\Model\Attribute\Backend\Data\Boolean::class,
                'visible'      => false,
                'position'     => 160,
                'user_defined' => true,
                'required'     => false,
                'system'       => false,
                'default'      => 0,
            ]
            **/
        );

        $attribute = $customerSetup->getEavConfig()->getAttribute(
            Customer::ENTITY,
            'my_custom_attribute'
        );

        $customerEntity = $customerSetup->getEavConfig()->getEntityType(Customer::ENTITY);
        $attributeSetId = $customerEntity->getDefaultAttributeSetId();
        $attributeSet = $this->attributeSetFactory->create();
        $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);

                /** @var mixed[] $rules */
        $rules = [
            'input_validation' => 'alphanumeric',
            'min_text_length'  => 10,
            'max_text_length'  => 15,
        ];
        $attribute->setValidationRules($rules);
        $attribute->addData(
            [
                'attribute_set_id' => $attributeSetId,
                'attribute_group_id' => $attributeGroupId,
                'used_in_forms' => ['adminhtml_customer', 'customer_account_create', 'customer_account_edit'], //define the areas where the attribute field should be displayed
            ]
        );

        $attribute->save();
    }
}