Most of these bikes have a reverse switch on the shifter side of the engine. You should find two wires that come out of the engine near the shifter. One is usually a neutral safety switch that indicates the bike is in neutral and is clear to start. The other is a reverse light wire that shows you on the dash that the bike is in reverse like @06kfx440 mentioned.
When you put the bike in reverse that switch is grounding the reverse light and makes it come on. So in theory all you would have to do is wire a reverse light as so, positive wire connected to the battery with an in-line fuse (must have for protection) and then attach the ground wire from the light to that reverse light wire coming from your neutral safety/reverse light switch. Of course you would have to look at your bike to make sure it works the way I’m stating or your manual to show what wire to connect to but a tester can do that for you also. But each manufacturer is different but mostly all of them use the grounding type system.
I agree with @06kfx440 about tapping your dash light but most of those indicator lights are super low amperage and may not carry the load of another light on the line.