Magento 2 : Create Your Own Customer Attribute

customerAttributeInstaller.php

<?php 
/**
 * @license http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 * @author Phuong LE <phuong.le@agence-soon.fr> <@>
 * @copyright Copyright (c) 2016 Agence Soon (http://www.agence-soon.fr)
 */

namespace NameSpace\Customer\Setup;

use Magento\Customer\Api\CustomerMetadataInterface;
use Magento\Customer\Model\Customer;
use Magento\Customer\Setup\CustomerSetup;
use Magento\Customer\Setup\CustomerSetupFactory;
use Magento\Eav\Model\Entity\Attribute\Set;
use Magento\Eav\Model\Entity\Attribute\SetFactory;
use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;

class InstallData implements InstallDataInterface
{
    /**
     * @var CustomerSetupFactory
     */
    private $customerSetupFactory;
    /**
     * @var SetFactory
     */
    private $attributeSetFactory;

    /**
     * InstallData constructor.
     * @param CustomerSetupFactory $customerSetupFactory
     * @param SetFactory $attributeSetFactory
     */
    public function __construct(
         CustomerSetupFactory $customerSetupFactory,
         SetFactory $attributeSetFactory

    ) {
         $this->customerSetupFactory = $customerSetupFactory;
         $this->attributeSetFactory = $attributeSetFactory;      
    }

    /**
     * Installs data for a module
     *
     * @param ModuleDataSetupInterface $setup
     * @param ModuleContextInterface $context
     * @return void
     */
    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
         /** @var CustomerSetup $customerSetup */
            $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);
            $customerSetup->addAttribute(Customer::ENTITY, 'my_custom_attribute',
                [
                    'type' => 'varchar',
                    'label' => 'My Custom Label Attribute',
                    'input' => 'text',
                    'required' => false,
                    'sort_order' => 30,
                    'visible' => true,
                    'system' => false, // Otherwise value won't be saved
                    'position' => 30,
                ]);
            /** Allow us to used in grid in backoffice */
            $customerSetup->updateAttribute(Customer::ENTITY, 'my_custom_attribute', 'is_used_in_grid', true);
            
            $attributeSetId = CustomerMetadataInterface::ATTRIBUTE_SET_ID_CUSTOMER; // The attribute set Id for customer is 1
            /** @var Set $attributeSet  */
            $attributeSet = $this->attributeSetFactory->create();
            $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);
            /** We need to declare in which forms we can fill our data custom attribute */
            $usedInForms = [
                'adminhtml_customer',
                'adminhtml_checkout'
            ];

            /** @var \Magento\Eav\Model\Entity\Attribute\AbstractAttribute $attribute */
            $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY,'my_custom_attribute');
            $attribute->addData([
                'attribute_set_id' => $attributeSetId,
                'attribute_group_id' => $attributeGroupId,
                'used_in_forms' => $usedInForms,
            ]);
            $attribute->getResource()->save($attribute);
    }
}