I have an action in one of my controllers that is going to receive HTTP POST requests from outside of my MVC website.
All these POST requests will have the same par
The web server shouldn't care where the request is coming from. If your client application has a input control called username and it posts to your application it will pick up the same as if your posted if from your own application with an input called username.
One huge caveat is if you have implemented AntiForgeryValidation which will cause a big headache to allow an outside form to post.