first, keep in mind that anything you modify in the contyroller could damage it and keep it from working at all, so:
if i had to guess i'd say you ground the abs pad to do regen braking. probably it will need to be grounded all the time, then you use yhour brake lever switches to engage it
it may not even support regen, not all controllers do
usually regen on these things is just on/off, so when you hit the brakes it will do whatever it's factory-designed to do; if hta'ts powerful it could lock up y our wheel. if it's a front hubmootor, that could flip the bike and send you flying over the bars.
some controllers respond differently at different speeds and different battery voltages, so it may not respond the same way in all situations, and you'l have to learn how it does what it does by experimentation.
if you have a battery with a separate port bms or no bms, regen will not be stoppable by the bms in the event of a battery problem, and the battery can be damaged, so you will need to monitor the process yourself to make sure it doesn't send back more current into the battery than the battery is able to handle for charging, or that it charges any cell higher than it's limts.
caution: if you have a battery with a common-port bms, and you're doing regen downhill with a full battery long enough to make the bms turn off the connection, the resulitng voltage spike in the controller can blow it up beofre you can stop braking.