Where can I hook up an electric choke?

I'd do what replicaracer suggest or run a relay like 67Dart suggest.

I don't like any significant load on the ignition power wires than necessary. A good relay will draw less than .25 amps, so trigger the relay, 85 terminal with a wire from the blue ignition wire. Run a fused power line to the 30 relay terminal and out to choke on 87 terminal. Ground the relay from 86 terminal. As Replicaracer mentions, the choke will be hot with key in run.