Atom APIs provided with IBM® Connections 4 allows to start following a resource included a Forum. Here's one easy way to do it with an asynchronous HTTP (Ajax) request using jQuery.
IBM Connections 4 documentation link: Start following a resource
API Request Details
|Response||Status Code: 200 OK|
Example using jQuery
Note: console.log can throw errors in IE, they are there for illustrative purposes only and should be changed as required.