The file which you need to edit is service_widget.php located in /inc/widgets/.

We will be adding this as a new feature in the next release of Integral, which will be in the next 1-2 weeks.

Either you can wait until then or you can edit the file yourself.

Here is a widget file which contains a link,

You can use this file to compare with the current service_widget.php and re-create the link feature.

You will see references to btn_link

Hope that helps for now.