You don't, at least not on the Free Hosting servers. You would need to be on Illuminated, Premium or VPs to have ASP.NET available, and then it will be using the
Mono runtime, which is not exactly the same as the .NET runtime on Windows servers. "Classic" ASP is not supported on any of x10's servers.
On Free Hosting (vanilla or Prime), the only server-side scripting language consistently available to you is PHP.
Or did you mean that you want to use Visual Studio as a text editor for a language it doesn't support? There are many more suitable editors for PHP, HTML, CSS and JavaScript available for free online, and if you don't mind paying a few bucks, JetBrains' PhpStorm (based on the IntelliJ platform, or IntelliJ IDEA with the PHP plugin) really can't be beat.