Although you could try to make it a calculated value, since the value is essentially going to be constant, I'd just compute it when the form opens/loads and put the value in there. So, if this textbox is named txtMaxDate, you could have
txtMaxDate.Value = DMax("visit_dt", "VISIT")
in the form's open or load event. If any actions on the form could possibly change the max value, then you might need to recompute it. But, if you do want to make it a calculated value, you can set its control source to
Hope that helps.