Hi,
Typically you will set registers exactly how you have done, by sending some specific command to to the interface that is only valid for that specific device. You typically have to consult documentation to see what specific commands correspond to the register you're trying to write to, I'm not familiar with Grove sensors, but it looks like their ADC module is based off of the ADC121C021 chip, which they have helpfully provided documentation for here : https://raw.githubusercontent.com/SeeedDocument/Grove-I2CADC/master/res/ADC121C021Datasheet.pdf
Consulting that document on page 23, you can see specifics of what commands are supposed to be used for this particular device. Hope this helps.
Thanks,
Ian