Navigate to my web site with Post method from Desktop application

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C#2.0

C#

Hi There

I seem to get nowhere. All over the help it always explains how to get data from a web page. I don't want to get data, I want to send it there as I navigate there from my Windows desktop application.
I want to be able to navigate to my web site, and also submit my post data, from a desktop application (only interested in C#). I do not want to read from the web site, I want to navigate to the web site but not use method get, as that would be easy but method post.

The client using the Desktop application will click a button on this desktop application, that will navigate to the web site with the post data, check his credentials and then if ok log him into the site.

I can do this:
System.Diagnostics.Process.Start("https://www.supremweb.com/taxi/buydriverdownloadversion.aspx?txtUserName=sad&txtpassword=happy");

Now this is easy but I want Post not Get. The method above is get.
I have no issues with getting the form data in C#.NET on the web site, but how would I include the Post data in it as I navigate to the site?

I don't want to see anything in the URL BAR of the browser, not even encrypted.

Many Thanks for anyone helping me on this.