Forms :: How To Make A Text Box Entry Required

I know how to make a text box entry "required" such that one cannot exit a record until a value is entered but cannot figure out how to make it required only if another field on the form has an entry. None of the form events seem to support that when focus moves to another record as far as I can tell. What I want to do is display an error message if the condition is not met on exiting a record but to also remain on (or go back to) that record so the required text box entry can then be made.