Type lookahead in To field on email memo form question

I’ve imported a local address book so that when I type in an email address in the to field on the memo form, it should find docs in that database, however, this is a custom database and not the standard local address book. How do I make it so that it will find these docs in the lookahead, i.e. are there certain fields I need on the form, or certain names/aliases I need to make it all work?

Thanks much