<dllimport("user32.dll")> _ 
  Friend Shared Function SetParent(ByVal hWndChild As Integer, ByVal hWndNewParent As Integer) As Integer 
  End Function 

  'To make the form a child, we need to use the SetWindowLong API function. But, of course, to use it, we need to declare it first. While we're at it, we will also declare a few constants. 
  'The constant GWL_STYLE is used to tell SetWindowLong that we will be changing the window style. We use the Or operator between the constants WS_CHILD and WS_VISIBLE, so that both these "styles" are set for the form's window: 

  Friend Const GWL_STYLE = (-16) 
  Friend Const WS_CHILD = &H40000000 
  Friend Const WS_VISIBLE = &H10000000 

  <dllimport("user32.dll")> _ 
  Friend Shared Function SetWindowLong(ByVal hwnd As Integer, ByVal nIndex As Integer, ByVal dwNewinteger As Integer) As Integer 
  End Function 



'1st create an instance of a form to be the child , eg called frmMain 

      Dim formHandle As Integer 

' Create a new window to be the parent - (this could be any form or window, not just a visio window) 
wAddon = myparentapp.App.ActiveWindow.Windows.Add("Parent Window", VisWindowStates.visWSDockedLeft Or VisWindowStates.visWSVisible, VisWinTypes.visAnchorBarAddon, , , 300, 210) 

'Get a handle to the form 
formHandle = frmMain.Handle.ToInt32 

' set the form's Child attrib so it can be a child 
retVal = SetWindowLong(formHandle2, GWL_STYLE, WS_CHILD Or WS_VISIBLE) 

'Now that we have set the form as a child and got the form handle, make the SetParent API call 
retVal2 = SetParent(formHandle2, wAddon.WindowHandle32)