Hello Jodie,
You can't force an entry here but you can come close. In the Order Entry template, define a non-existent salesperson as the default salesperson. If you try to leave the header screen without changing it, you will get an error message asking you to choose a valid salesperson. Since it is easier to select a valid entry than to highlight the non-existent code and hit delete, the entry will probably never be left blank.
Rick McLeod
Sunbeam Specialties Inc.