I would like to, by clicking a menu button in Word, launch
a URL that should open within the frame of an already open
browser window (frameset).

Currently the Visual Basic macro uses the
InternetExplorer.Application OBJECT, its Navigate METHOD,
and the TargetFrameName ARGUMENT to open the URL.

Unfortunately, the URL is NOT being opened within the
specified frame in the already open browser window.
Instead, it is opening in a new browser window.

I am using the following code:
Dim TestBrowser As Object
Set TestBrowser = CreateObject
("InternetExplorer.Application")
TestBrowser.Visible = True
TestBrowser.Navigate URL_To_Launch, Flags = Null,
TargetFrameName = "_Main", PostData = Null, Headers = Null

Your comments are greatly appreciated!

Arturo