Try this code in your UserForm:

Private Sub CommandButton1_Click()
xx = Mid(TextBox1.Value, 1, 2) * 1
yy = Mid(TextBox1.Value, 3, 2) * 1
'Note: * 1 converts strings to numbers
'To Do: limit xx & yy to valid range
ActiveSheet.Cells(xx, yy).Formula = TextBox2.Value
Unload Me
End Sub