Magento 2 - How to save custom filed data of registration page?

Question

Please see this screenshot,

I try with Install Data

<?php

namespace CP\Customerattribute\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;

/**
* Install data
* @codeCoverageIgnore
*/
class InstallData implements InstallDataInterface
{
    /**
    * CustomerSetupFactory
    * @var CustomerSetupFactory
    */
    protected $customerSetupFactory;

    /**
    * $attributeSetFactory
    * @var AttributeSetFactory
    */
    private $attributeSetFactory;

    /**
    * initiate object
    * @param CustomerSetupFactory $customerSetupFactory
    * @param AttributeSetFactory $attributeSetFactory
    */
    public function __construct(
        CustomerSetupFactory $customerSetupFactory,
        AttributeSetFactory $attributeSetFactory
    ) {
        $this->customerSetupFactory = $customerSetupFactory;
        $this->attributeSetFactory = $attributeSetFactory;
    }

    /**
    * install data method
    * @param ModuleDataSetupInterface $setup
    * @param ModuleContextInterface $context
    */
    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);
        /**
        * customer registration form default field mobile number
        */
        $customerSetup->addAttribute(Customer::ENTITY, 'custom_field', [
            'type' => 'varchar',
            'label' => 'ID Documents',
            'input' => 'text',
            'required' => true,
            'visible' => true,
            'user_defined' => true,
            'sort_order' => 1000,
            'position' => 1000,
            'system' => 0,
        ]);
        //add attribute to attribute set
        $attribute = $customerSetup->getEavConfig()->getAttribute(Customer::ENTITY, 'custom_field')->addData([
                'attribute_set_id' => $attributeSetId,
                'attribute_group_id' => $attributeGroupId,
                'used_in_forms' => ['adminhtml_customer', 'customer_account_create'],
          ]);
        $attribute->save();
    }
}**

Answer 1

You can override this controller Magento\Customer\Controller\Account\CreatePost with di.xml

and Below code will help you to save custom attribute value

  <?php
    public function __construct(
        \Magento\Customer\Api\CustomerRepositoryInterface $customerRepository,
        \Magento\Framework\App\Action\Context $context
    )
    {        
        $this->customerRepository = $customerRepository;
        parent::__construct($context);
    }

    public function execute(){
        $customerId = XX;
        $customer = $this->customerRepository->getById($customerId);

        $customer->setCustomAttribute('custom_attribute!','value for attribute');
       try {                
            $customer = $this->customerRepository->save($customer);
        }catch (Exception $e) {
            return $e->getMessage();
        }
    }

So what we did here, in Magento 2 we have to user setCustomAttribute method to save custom customer attribute.

For overriding that controller you may refer to this post https://magento.stackexchange.com/a/154517/49821

Answer 2

Add additional field in a customer account.

Check the reference link Reference Link