We have removed the previous functionality of adding a link to a Call Activity. In the current version of Cawemo, you can still add links by pasting the URL to the specification text of a Call Activity.
The reason for this change was that we added a completely new way of how these links work to align it with how the Camunda Engine executes such Call Activities. This is also why we decided to turn this into an Enterprise-only feature.
Please let us know if you have any further questions to this.