You will need to perform some basic steps to create and set a new attribute on the customer model.

• Create a new module, with a setup script, with the following files:
  • app/code/{CompanyName}/{ExtensionName}/registration.php
    
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    <?php \Magento\Framework\Component\ComponentRegistrar::register(     \Magento\Framework\Component\ComponentRegistrar::MODULE,     'CompanyName_ExtensionName',     __DIR__ );

  • app/code/{CompanyName}/{ExtensionName}/etc/module.xml
    
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    <?xml version="1.0"?> <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">     <module name="{CompanyName}_{ExtensionName}" setup_version="1.0.0">         <sequence>             <module name="Magento_Customer"/>         </sequence>     </module> </config>

  • app/code/{CompanyName}/{ExtensionName}/Setup/InstallData.php
    
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    <?php   namespace CompanyName\ExtensionName\Setup;   use Magento\Customer\Setup\CustomerSetupFactory; use Magento\Customer\Model\Customer; use Magento\Eav\Model\Entity\Attribute\Set as AttributeSet; use Magento\Eav\Model\Entity\Attribute\SetFactory as AttributeSetFactory; use Magento\Framework\Setup\InstallDataInterface; use Magento\Framework\Setup\ModuleContextInterface; use Magento\Framework\Setup\ModuleDataSetupInterface;   /** * @codeCoverageIgnore */ class InstallData implements InstallDataInterface {       /**      * @var CustomerSetupFactory      */     protected $customerSetupFactory;       /**      * @var AttributeSetFactory      */     private $attributeSetFactory;       /**      * @param CustomerSetupFactory $customerSetupFactory      * @param AttributeSetFactory $attributeSetFactory      */     public function __construct(         CustomerSetupFactory $customerSetupFactory,         AttributeSetFactory $attributeSetFactory     ) {         $this->customerSetupFactory = $customerSetupFactory;         $this->attributeSetFactory = $attributeSetFactory;     }         /**      * {@inheritdoc}      */     public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)     {           /** @var CustomerSetup $customerSetup */         $customerSetup = $this->customerSetupFactory->create(['setup' => $setup]);           $customerEntity = $customerSetup->getEavConfig()->getEntityType('customer');         $attributeSetId = $customerEntity->getDefaultAttributeSetId();           /** @var $attributeSet AttributeSet */         $attributeSet = $this->attributeSetFactory->create();         $attributeGroupId = $attributeSet->getDefaultGroupId($attributeSetId);           $customerSetup->addAttribute(Customer::ENTITY, 'attr_name', [             'type' => 'varchar',             'label' => 'Your attribute label',             'input' => 'text',             'required' => false,             'visible' => true,             'user_defined' => true,             'sort_order' => 1000,             'position' => 1000,             'system' => 0,         ]);   /* used_in_form should be set wherever you want to see your attribute, they can also be:       adminhtml_checkout     adminhtml_customer     adminhtml_customer_address     checkout_register     customer_account_create     customer_account_edit     customer_address_edit     customer_register_address */         $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'attr_name')             ->addData([                 'attribute_set_id' => $attributeSetId,                 'attribute_group_id' => $attributeGroupId,                 'used_in_forms' => ['adminhtml_customer'],             ]);           $attribute->save();         } }

  • app/code/{CompanyName}/{ExtensionName}/view/adminhtml/ui_component/customer_form.xml
    
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    <?xml version="1.0" encoding="UTF-8"?> <form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">     <fieldset name="customer">         <field name="attr_name" formElement="input">             <settings>                 <visible>true</visible>             </settings>         </field>     </fieldset> </form>

    
    Note: In contrast to Magento 1, in Magento 2 you now need to add some XML to make custom attributes appear in the edit customer section of the admin.

Afterwards, and before testing, remember to enable the module, clean cache, and do a setup:upgrade.