Let's put it this way.
If you, the employer, and the medical condition all qualify for FMLA, the employer MUST offer and and MUST at least provisionally designate FMLA, whether you request it or not; whether you want it or not.
However, short of holding a gun to your head, they cannot force you to return the medical certification. You are free to withhold it if you are determined not to do so.
The flip side of this, though, is that if you do not return the certification within the time specified (by law, it cannot be less than 15 days - it can be more if the employer so states), then they can pull the provisional FMLA and its protections. That would mean that you are subject to the regular company attendance policy and can be fired for excessive absences.
What difference does it make whether you requested it or not? If you don't sign it, you don't get the leave.
Two things I am tired of typing: 1.) A wrongful termination does not mean that you were fired for something you didn't do; it means that you were fired for a reason prohibited by law. 2.) The above answer, whatever it is, assumes that no legally binding contract or CBA expressly says otherwise. If it does, the terms of the contract apply.