I hope you're doing something within the acceptable terms of use with the content you automatically post. Ie. you do not ask how to create yet another spam bot...
To grab the HTTP form you can use WebRequest. This returns the content of the page (including the form) as a response stream. You can then parse the response using HtmlAgility pack, for the forms you are interested. Once you know the forms and fields in the page, you can set values for the fields and post a response, again using a WebRequest but changing the method to POST and encoding the reponse fields as application/x-www-form-urlencoded content, see How to: Send Data Using the WebRequest Class.
This method is using almost the most basic building blocks, going lower level than this would mean using sockets and formating the HTTP request yourself. At this low level you'll have a great deal of freedom and flexibility on how to parse the form and send back the request, at the cost of actually having to understand how WebForms and HTTP work.