ajax

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 will ajaxify the respective element and will trigger an event. Respective action will be taken by the code and displayed on the form within the specified wrapper

This CodeLet let you konw how to implement ajax on field group multiple module.
"fgm_node_YOURNODE_form_group_YOURGROUPNAME" where "YOURNODE" will be the name of your content type and "YOURGROUPNAME" will be the name of your field group,
Given example is for text field, ajax call on blur event.