VBA's OnAction Method

[Keidi]

I am trying to add a button to outlook's tool bar using a Macro.The problem is that the button appears but on clicking it nothing happens,I suspect I am supplying parameters wrongly to the OnAction method.
The call

Call AddToolbarButton("My very own button", "Click here", "!<Sub/UserForm>")

The method it's self:

Function AddToolbarButton(Caption As String, _
                       toolTip As String, macroName As String, _
                       Optional toolbarName As String = "Standard", _
                       Optional FaceID As Long = 325)
                       
Dim objBar As Office.CommandBar
Dim objButton As Office.CommandBarButton
Set objBar = ActiveExplorer.CommandBars(toolbarName)
Set objButton = objBar.Controls.add(msoControlButton)

  With objButton
    .Caption = Caption
    .OnAction = macroName
    .TooltipText = toolTip
    .FaceID = FaceID
    .Style = msoButtonIconAndCaption
    .BeginGroup = True
  End With
  
End Function

Any help would be highly appreciated.