How do get mestic ticket in Pokemon FireRed?

You can't get a Mistic Ticket in Pokémon FireRed. The Mistic Ticket is a special item introduced in Pokémon Emerald and its remakes, Omega Ruby and Alpha Sapphire. It allows the player to access the Battle Frontier, a post-game area with challenging battles and rewards.

FireRed, being an older game, doesn't have the Battle Frontier, so the Mistic Ticket isn't available.

If you're looking for a special item in FireRed, perhaps you're thinking of a different item?