You have a few choices, though the best one might be to bring the bike inside while you work.

However, if the parking area is relatively secure, you can lock the bike to the car with a cable. One way is to run the cable to a chassis element of the car, ie the tie down loops on imports. Another way is to improvise a bent piece of sheet metal that can be trapped under the trunk lid (often one of the hooks like what's holding your rack on, can serve, but this needs to be checked).

Then you can lock the bike to the trapped anchor.

Depending on the area, I'd add a secure lock to the bike itself, ie. a U-lock holding the wheel to the frame, so the bike won't be as tempting as a quick grab and ride opportunity.