No. in the form properties, in the on-load event, set the reference number control to disabled. In the properties for the yes control, in the after update event, indicate an if-then statement that if the control value is Yes, then set the reference number control to enabled.
See below examples
Code:
Option Compare Database
Option Explicit
Private Sub Field5_AfterUpdate()
If Me.Field5.Value = "YES" Then
Me.Field1.Enabled = True
End If
End Sub
Private Sub Form_Load()
Me.Field1.Enabled = False
End Sub