Private Sub Form_KeyDown(KeyCode As Integer, Shift As Integer)
If KeyCode = vbKeyF9 Then
    Call Command1 'Aqui tambien puedes poner el codigo para que el timer se encienda (Timer1.Start) y que el timer tenga el codigo deseado.
    Else
Call Command2 'O sino Timer1.Stop
 End If
 
Private Sub Form1_Load()
Me.KeyPreview = True
End Sub

Private Sub Form_KeyDown(KeyCode As Integer, Shift As Integer)
If KeyCode = vbKeyF9 Then
Call Command1 'O sino el Timer1.Start
End If
 
If KeyCode = vbKeyF8 Then
Call Command2 'O sino Timer1.Stop
End If