If I were to do that, I would create a html form in my asp page with hidden user/password fields and hidden/visible XML input fields and action as your URL, and use POST method to get the response.
However if you want to create a link then you could use the following URL:
We are also searching for a sample code on invoking webMethods flow service from ASP (Http POST). It is highly appreciated, if some one helps us with sample code.