autofill "@domain.com" in a text box

I created on online access form to gain access to a web site on my company intranet. In the form we ask for both the user's & their manager's email address.
What I want to do is similar to what Microsoft has done with their Hotmail login screen. There, a user types their username and when you press Tab, it automatically fills in @hotmail.com.
So, what I am wanting to do is add this type of function to my form, so that when they begin typing and press tab, that it fills in @domain.com, "domain" being replaced by our company name.
Does anyone know how to write the code to do this or if it can be done? I'm not using it for a login page, I just need it to autofill a textbox.

Thanks
MT