How do you get the HM Surf in Pokémon brick bronze?

You can't get the HM Surf in Pokémon Brick Bronze.

Pokémon Brick Bronze is a fan-made game, and the developers have chosen to not include certain features from the main series games, such as HMs.

This means there is no HM Surf, and you won't be able to access water routes using Surf in the game.