Any documentation on getting the DialogWindow "?" help button to send the user to non-MSDN help for a VS 2015 extension?

I am developing a VS 2015 extension which presents a dialog box to the user, derived from the Microsoft.VisualStudio.PlatformUI.DialogWindow class. I know how to get the dialog to show the "?" help button next to the close button, providing and calling an overloaded constructor and passing in a string which is documented as the help topic.

    public partial class MainDialogWindow : DialogWindow
    {      
        public MainDialogWindow(string helpTopic) : base(helpTopic)
        {

I have not been able to find documentation on exactly what the contents of the string should be, or how (if it's even possible) to open a web page that is somewhere other than on the MSDN site.  Based on experimentation, I have found that I am taken to a generic VS 2015 welcome page, unless the string happens to result in a successful search within MSDN (e.g., "Microsoft.VisualStudio.PlatformUI.DialogWindow" will end up on the documentation page for that class). Just for fun, I tried passing in a URL of a page containing the help for my dialog, but ended up at the generic VS 2015 welcome page. Of course, I'm interested in providing help for my specific dialog for my specific extension, which is not going to be on MSDN.

Is there any documentation on how to gain control of where the user is sent when the "?" help button is clicked in a DialogWindow-based dialog box?  Alternatively, is there an event I can use to capture and handle the click on the "?" help button, so that I can implement my own help handling and bypass the MSDN search behavior?  I looked through all the events for this class, and didn't see one for this.

It seems like this should be straightforward, so that an extension can provide its own help from an extension-specific DialogWindow when the user presses the "?" button in the title bar. I'm just not seeing it.

Thanks.