form

This CodeLet set title and body field values using Ajax. 
Place this CodeLet to your custom module file(.module) or Create a Custom module for Drupal 8 as per documentation https://www.drupal.org/docs/8/creating-custom-modules.

First of all add a field(field_link) to your content type. after that add an ajax to that field as per CodeLet.
Ajax example to replace multiple fields on ajaxCallback.
You set multiple fields value on ajax trigger in Drupal 8.

This CodeLet let you programmatically create a form page and render same form on a block. Create custom module and follow CodeLet instruction to create a form and block programmatically.