If she is working on the spaceship itself then she is a part of the crew in that case the captain/crew commander would discuss with the rest of the vessel's medics who to get in if a spot opened on the ship in the medical crew. She would also need a ISA approval for traveling beyond Moon (basically ISA dues a series of physical and mental elevations if one is capable of traveling beyond Moon).
She would start as a Junior Medical Expert and if shown capable after a few years promoted into Senior Medical Expert. To get to be the leader of the Medical part of the vessel's crew she would need to be to voted to become the leader by 2/3 of the Medical Experts plus the captain/crew commander.