There is no reward. It is an electoral process. You don't get paid for this vote. You wouldn't ask to get paid to vote on your president because that would invalidate the election.

Here is an explanation of witness voting
https://steemit.com/life/@socky/witness-system-explained-simply

I will wait to add you to our list until you have read and understand what witness voting is. If you still want to participate, come back and let me know that you want to promise to vote.