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, https://www.dropbox.com/s/s95bas6disydnu4/feature-widget.php?dl=0
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.