That would depend on what type of field. If it is a content control in the document itself then something like this
Code:
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Select Case ContentControl.Title
Case "ID"
If Not ContentControl.Range.Text Like "####-###-####" Then
If IsNumeric(ContentControl.Range.Text) And Len(ContentControl.Range.Text) = 11 Then
ContentControl.Range.Text = Format(ContentControl.Range.Text, "####-###-####")
Else
MsgBox "Please enter an eleven digit ID"
End If
End If
End Select
End Sub