There are two webmail clients included with DirectAdmin, and another available through a script. Also others you can install yourself.
First, if you're using DirectAdmin, then youv'e asked a fair question, but unless you tell us which email client you're using, we can't even guess, as each webmail client manages its address book differently.
Second, even once you do tell us, unless one of us has done it before we're probably going to tell you that you should ask the support group for the webmail client you're using.
Jeff