How do I accept and return string over an empty webpage?

I'm working on a system that has to accept string from a webpage that is owned by another company. Due to policies this happens over a webpage. However this specific web page has no actual content except for the data that has is returned from my code and the input from the other company. So I was planning on using POST and GET to get this done but seeing as how I have little experience on web programming I couldn't find how to do this.

So what is a good way to receive and send a string from an empty webpage using c# webforms?