Pro Bono work for another organization (non-profit, free, part-time)

Can Someone advise if it is possible to work for another organization than your H1-B sponsoring organization on a part time basis (3-5 hours per week) for no pay (pro bono)?

If you’re doing Social Service, and I mean “Social Service” for a not-for-profit organization without compensation AND your current Employer doesn’t have a problem with it, then it should be fine.

Suggest you to read the leaflet given by US Visa Officer that explains your rights well before and then run it through with your employer for consent.

Thanks for the answer.
So I need written consent from my current employer even if I am doing social service for a non-profit organization for free in my spare time.
Is this leaflet available online? Would it be possible for you to point me towards it?
Thanks so much!