Sadly,you usually don't. Often,if it takes persuasion to have a pet, parents have something against getting pets. Something like rats, many take as just vermin dirty creatures even though pet rats are far form that. Might be other issues with parents knowing your responsibility level.
Last thing parents want is to care for some animal you may tire of or decide their care is too much work.
My parents were a bit nature and animal lovers, we had a number of farm sort animals, a dog, couple cats. Didn't really mind my keeping small animals, mostly bugs in younger years.
I rescued a baby orphaned mouse, first mom said no,but couldn't see just killing it. when she caved, the rule was if it smelled, it was gone.
Turned out to be the best pet ever and when my parents saw my dedication and responsibility, ratties were no problem. Now I presume you know you should keep rats in pairs or more, and they can live 3 years or better so this is a commitment for the long run.
If parents are just against rats, have patience, wait till you have your own place, then it is your call. You will still want them, I have had dozens across decades and still would keep them.