The short answer: an illegal saloon.
Even in pre-prohibition days, you had to have a license to sell liquor, but there were folks who found various ways to attempt to avoid that.
People would open “museums” or some other fake front, and charge admission. And with that admission charge, there were free drinks.
Think of carnival tent where they're offering you tickets to see a tiger which is blind, and you get a free drink inside. You can also hear such illegal saloons referred to as blind pigs. Same deal, different name.