I'm assuming you mean an input mask. If so, I would recommend using a jQuery input mask like so:
If you really want to use an ASP.NET control to do the input mask for you, here is a possible solution for you:
I would recommnd against the second solution because it is heavier and not as clean as the jQuery method. jQuery is designed for client-side work, whereas ASP.NET is mainly server-side technology.
If you mean you don't want to show the characters that are being typed, you need to use the input type of password like so:
<input type="password" />