MSHTML and HTML Agility Pack are two very different things.
MSHTML is a rendering engine that provides interactivity with HTML.
Html Agility Pack is a parsing engine that provides the html nodes as data.
In HAP there are no events, no clicking. Even when using HAP as a COM object embedded in a web page as script I can't think of a way that you could initiate on onclick event or code.
If you're looking to parse html and then execute it I think it might be better to do something as a firefox extension. But without knowing the context of the program or the overall goal it is hard to say.