TOOLS > Options > Firm/ Preparer Info to set up your preparers and give them a preparer code.  That code needs to be entered in the Fed Info worksheet (Part V) for each client for that preparer's info to appear. on the return.

To set it up that the preparer is automatically the 3rd party designee, you need to be within a client file (not Homebase), then go to Tools>Options > Miscellaneous and scroll way down to the bottom of that window and check the box.