Mandatory prefix on asp.net form

I have a textbox receiving input, and I want to both display a single prefix character, but also prevent the user from erasing it in the textbox.  I'm more familiar with win form development, and know how I'd handle it there.
Is that going to be too complicated in asp.net?  I could simply include it myself, stripping it off if it's included, so I didn't want to go for any complex solution.