Don’t.
It’s not expected, and they don’t respond positively when you try. I tried the first night when I arrived and my cab driver talked to my contact on the phone to get me to the right place and waited with me until she showed up. He didn’t seem to understand and didn’t thank me. I haven’t since tried, and I’ve heard that it’s treated as if it were charity, and people don’t like to accept charity. So just pay what you are charged and that’s it.