1. Create di.xml at app/code/Magemonkeys/AjaxNewsletter/etc/di.xml folder.

   <?xml version="1.0"?>
   <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
       <type name="MagentoNewsletterControllerSubscriberNewAction">
           <plugin name="Newsletter_Subscriber_NewAction"
                   type="MagemonkeysAjaxNewsletterControllerPluginSubscriberNewAction" sortOrder="10" disabled="false" />
       </type>
   </config>

2. create NewAction.php at app/code/Magemonkeys/AjaxNewsletter/Controller/Plugin/Subscriber folder

   <?php
   /**
    *
    */
   namespace MagemonkeysAjaxNewsletterControllerPluginSubscriber;
    
   use MagentoCustomerApiAccountManagementInterface as CustomerAccountManagement;
   use MagentoCustomerModelSession;
   use MagentoCustomerModelUrl as CustomerUrl;
   use MagentoFrameworkAppActionContext;
   use MagentoStoreModelStoreManagerInterface;
   use MagentoNewsletterModelSubscriberFactory;
    
   /**
    * Class NewAction
    */
   class NewAction extends MagentoNewsletterControllerSubscriberNewAction
   {
       /**
        * @var CustomerAccountManagement
        */
       protected $customerAccountManagement;
    
       protected $resultJsonFactory;
    
       /**
        * Initialize dependencies.
        *
        * @param Context $context
        * @param SubscriberFactory $subscriberFactory
        * @param Session $customerSession
        * @param StoreManagerInterface $storeManager
        * @param CustomerUrl $customerUrl
        * @param CustomerAccountManagement $customerAccountManagement
        */
       public function __construct(
           Context $context,
           SubscriberFactory $subscriberFactory,
           Session $customerSession,
           StoreManagerInterface $storeManager,
           CustomerUrl $customerUrl,
           CustomerAccountManagement $customerAccountManagement,
           MagentoFrameworkControllerResultJsonFactory $resultJsonFactory
       ) {
           $this->customerAccountManagement = $customerAccountManagement;
           $this->resultJsonFactory = $resultJsonFactory;
           parent::__construct(
               $context,
               $subscriberFactory,
               $customerSession,
               $storeManager,
               $customerUrl,
               $customerAccountManagement
           );
       }
    
       /**
        * Retrieve available Order fields list
        *
        * @return array
        */
       public function aroundExecute($subject, $procede)
       {
           $response = [];
           if ($this->getRequest()->isPost() && $this->getRequest()->getPost('email')) {
               $email = (string)$this->getRequest()->getPost('email');
    
               try {
                   $this->validateEmailFormat($email);
                   $this->validateGuestSubscription();
                   $this->validateEmailAvailable($email);
    
                   $status = $this->_subscriberFactory->create()->subscribe($email);
                   if ($status == MagentoNewsletterModelSubscriber::STATUS_NOT_ACTIVE) {
                       $response = [
                           'status' => 'OK',
                           'msg' => 'The confirmation request has been sent.',
                       ];
                   } else {
                       $response = [
                           'status' => 'OK',
                           'msg' => 'Thank you for your subscription.',
                       ];
                   }
               } catch (MagentoFrameworkExceptionLocalizedException $e) {
                   $response = [
                       'status' => 'ERROR',
                       'msg' => __('There was a problem with the subscription: %1', $e->getMessage()),
                   ];
               } catch (Exception $e) {
                   $response = [
                       'status' => 'ERROR',
                       'msg' => __('Something went wrong with the subscription.'),
                   ];
               }
           }
    
           return $this->resultJsonFactory->create()->setData($response);
       }
    
   }

3. Make a copy of “vendor/magento/module-newsletter/view/frontend/templates/subscribe.phtml” and place it inside your Magento Theme (e.g. app/design/frontend/NAMESPACE/MY_CUSTOM_THEME/Magento_Newsletter/templates/subscribe.phtml
   Add the following javascript to the end of the subscribe.phtml:

   {
           "*": {
               "js/newsletter_subscriber_ajax": { }
           }
       }
   

4. Create newsletter_subscriber_ajax.js at app/design/frontend/NAMESPACE/MY_CUSTOM_THEME/web/js/ folder

require.config({
    deps: [
        'jquery'
    ],
    callback: function ($) {
        var form = $('form.subscribe');
 
        form.submit(function(e) {
            if(form.validation('isValid')){
                var email = $("#newsletter").val();
                var url = form.attr('action');
                var loadingMessage = $('#loading-message');
 
                if(loadingMessage.length == 0) {
                    form.find('.control').append('<div id="loading-message" style="display:none;padding-top:10px;">&nbsp;</div>');
                    var loadingMessage = $('#loading-message');
                }
 
                e.preventDefault();
                try{
                    loadingMessage.html('Submitting...').show();
                    $.ajax({
                        url: url,
                        dataType: 'json',
                        type: 'POST',
                        data: {email: email},
                        success: function (data){
                            if(data.status != "ERROR"){
                                $('#newsletter').val('');
                            }
                            loadingMessage.html(data.msg);
                        },
                        complete: function(){
                            setTimeout(function(){
                                loadingMessage.hide();
                            },5000);
                        }
                    });
                } catch (e){
                    loadingMessage.html(e.message);
                }
            }
            return false;
        });
    }
})