How do I change my screen name in an instant message to make it look like a different name? Ithinkitsawebsite?

I knew a person who could IM me and make it look like someone else's screen name does anyone know how to do it? I think it's a website but I can't find itHow do I change my screen name in an instant message to make it look like a different name? Ithinkitsawebsite?
you can't