How to Validate Admin Form using Ajax in Magento2

Here we will learn, how to validate admin form using ajax in Magento2

1) Create a getValidationUrl() function in your admin form Edit.php Block for example.
Here is Block: Webkul/FormValidate/Block/Adminhtml/CustomForm/Edit.php

<?php

namespace Webkul\FormValidate\Block\Adminhtml\CustomForm;

class Edit extends \Magento\Backend\Block\Widget\Form\Container
{
    /**
     * Block group name
     *
     * @var string
     */
    protected $_blockGroup = 'Webkul_FormValidate';

    /**
     * @param \Magento\Backend\Block\Widget\Context $context
     * @param \Magento\Framework\Registry $registry
     * @param array $data
     */
    public function __construct(
        \Magento\Backend\Block\Widget\Context $context,
        array $data = []
    ) {
        parent::__construct($context, $data);
    }

    /**
     * @return void
     */
    protected function _construct()
    {
        parent::_construct();
    }

    /**
     * Retrieve URL for validation
     *
     * @return string
     */
    public function getValidationUrl()
    {
        return $this->getUrl('/*/validate', ['_current' => true]);
    }
}

2) Create Validation Controller : Webkul/FormValidate/Controller/Adminhtml/CustomForm/Validate.php

<?php

namespace Webkul\FormValidate\Controller\Adminhtml\CustomForm;

class Validate extends \Magento\Backend\App\Action
{
    /**
     * @var \Magento\Framework\Controller\Result\JsonFactory
     */
    protected $resultJsonFactory;

    /**
     * Constructor
     *
     * @param \Magento\Backend\App\Action\Context $context
     * @param \Magento\Framework\Registry $coreRegistry
     * @param \Magento\Framework\View\Result\PageFactory $resultPageFactory
     * @param \Magento\Framework\Controller\Result\JsonFactory $resultJsonFactory
     */
    public function __construct(
        \Magento\Backend\App\Action\Context $context,
        \Magento\Framework\Registry $coreRegistry,
        \Magento\Framework\View\Result\PageFactory $resultPageFactory,
        \Magento\Framework\Controller\Result\JsonFactory $resultJsonFactory,
    ) {
        parent::__construct($context);
        $this->resultJsonFactory = $resultJsonFactory;
    }

    /**
     * @return \Magento\Framework\Controller\ResultInterface
     */
    public function execute()
    {
        $response = new \Magento\Framework\DataObject();
        $response->setError(false);

        $response->setMessage(
            __('Validate Successfully.')
        );
        return $this->resultJsonFactory->create()->setJsonData($response->toJson());
    }
}