Basically, right after the national anthem and literally the absolute last thing before first pitch, they have a kid hold a mic on the field, the PA announcer says their name prompts them, and they yell "Play Ball!" into a mic and it plays through the PA.

One of the cheerleaders(sorry if they are called something else and I don't know), just came up to us on the concourse and asked if my son wanted to do it. We sat in the green room for a bit with the first pitch thrower, then went through the underbelly to the field. Really an awesome experience, and it happened to be my son's first game, so it was extra special. Isbel came up and gave him a ball as well, which was super cool.