BetMGM has a couple of ways of getting free bets:
Sign Up Offer - Usually a risk free bet offer, BetMGM almost always offers free bets of some sort as a way to unlock free bets
Rewards Program - Through BetMGM's Monday Money Club, you can get granted a free bet every week just for playing regularly
BetMGM is really good about ensuring both new and returning customers are treated well and are able to receive free bets regularly.