But you must MAKE SURE that the text fields matches EXCACTLY the service name (services don't have searchcode).
Construct the UI/DB rule as usual. Select data update and from there select Service;Name. Set it to the vaule of the text field. Press ok to save it. Reopen the action. Hilight the ";Name" (after Service) and press Delete. You will see that the value is emtpy. Press Cancel. If you see the text of the rule it will be
Service set to [NAME OF THE FIELD]
If the text value doesn't match a service name you will get errors at the logs.