You have the right to make them explain exactly what the medication does, why they want to prescribe it, and whether there are other options. In fact, you have the right not to take medication, but I encourage you not to immediately discard the idea. I have ADHD+autism, as well, and I benefit from an extended-release stimulant for ADHD. It helps me stay better organized, and it makes it a little easier to switch tasks.
They should be prescribing the medication with the fewest side effects, at the lowest dose it can be while still being effective. Some autistic people do find that lower doses than usual are effective for them.
If you are going to try medication, ask them what improvements you should expect, and how long it will take to figure out whether the medication will help. If that time passes and you don't get improvements, or if you have side effects that are not worth the improvements, then you should go back to the doctor and talk about other options, either other meds, a lower dose, extended release, and/or adding various types of therapy. The first medication you try is not always the one that works for you; your brain is unique to you and works slightly differently from everyone else's, so doctors often have to do trial-and-error to figure out what works. It's annoying, but it's worth it once you do.