Outlook Template Sum TextBoxes

I have made a template that has several textboxs.  txt1, txt2, txt3

What I would like to do is sum txt1 + txt2 and have that display in txt3

I can do the below in a userform but I am not sure how to get what I need it a template

Private Sub TextBox62_Change()
Call totalTextBoxes
End Sub

Private Sub TextBox63_Change()
Call totalTextBoxes
End Sub

Private Sub totalTextBoxes()
Dim RunningSum As Double
RunningSum = 0
With Frame3
    RunningSum = RunningSum + Val(.TextBox61.Value)
    RunningSum = RunningSum + Val(.TextBox62.Value)

  .TextBox64.Text = CStr(RunningSum)
End With
End Sub

Jason