Use the AfterUpdate event procedure of the text box to create a string to
assign to its DefaultValue property:
Private Sub MyDate_AfterUpdate()
Me.MyDate.DefaultValue = """" & DateAdd("d", 14, Me.MyDate) & """"
End Sub
DateAdd() adds the 14 days.
The quote marks are needed to turn the result into the string.
The default value is seen when you move to the new record.