Standing on the shoulders of giants. I have converted the code from the below mentioned sites to work with CakePHP 3.

• Normal database backed Form with AJAX JQuery code
  • Inspired by: http://jamnite.blogspot.com.au/2009/05/cakephp-form-validation-with-ajax-using.html
  • Example here: https://jmits.com.au/cake3/posts
• Modelless Form with AJAX JQuery code
  • Inspired by: http://miftyisbored.com/a-complete-tutorial-on-cakephp-and-ajax-forms-using-jquery/
  • Example here: https://jmits.com.au/cake3/

Code here => https://github.com/jmcd73/cakephp3-ajax-samples