It's done on the W-2 interview entry screen in the Income topics section. Does TurboTax think you were finished, so your return interview is closed, and you don't know how to get back to it? If so, at the Tax Home click on "Add a State" to reopen the interview (that doesn't really add one.)
The Utah state employer withholding ID must be 14 characters--11 digits followed by wth in lower case. Example 12345678901wth and the wth must be lower case.
For mineral withholding, the ID ends in wmp (again in lower case.)