A BNC to SMA connector, obviously. If you search Google, you'll find a bunch.
What you didn't ask was, is it a good idea to use an external antenna on a handheld? In most cases, no. The problem is, most handhelds have a large amount of frontend gain to make up for their poor antenna. Connecting one to a good antenna (which a mag mount isn't) just causes an inordenant amount of intermod. This is why most Elmers don't advise buying a handheld as a first radio.