For most users it will not be possible to determine what the ID should be. The web services should accept the username as well. This will make the SBM orchestration work much easier.
Q. Why do I see this error in the orchestration debug log? Please select a valid value for the "Team Lead" field. (TTwrnIDS_F1_MSG_FIELDS_INVALIDSELECTION) or Unable to complete transition of item. One or more fields are invalid.