Get user-highlighted text from a textbox in C# web

I have a web app in which I want to be able to highlight a word or two in a multi-line textbox, and have a button click method manipulate the selected text. I want to be able to store the selected text in a variable but I haven't been able to find a way to get highlighted text in System.Web. I know that System.Windows textboxes have the SelectedText property which would be perfect, but unfortunately I don't know how to use anything but the asp.net framework with html pages and c# code behind.

Basically what I am after is an equivalent of TextBox.SelectedText that I can use in a web app, or an alternate route which will accomplish the same result.