I recently took the TOEFL test, like less than a month ago, and didn't mention my stutter because it would take way too long for them to adjust the exam to my needs (I think a min of 6 weeks) and I was against the clock, so I decided to simply hope for the best. My speaking score wasn't nearly perfect (23) but I don't know if this was because of how much I stuttered or because I didn't answer the questions correctly. However, I got an overall really good score, so in my case it was fine really. If you feel like it definitely will negatively affect your score though, you have to mention it several weeks beforehand and need to have some kind of proof, like a medical certificate. But yes, you do need to tell them and they definitely will help you, unfortunately I don't know how, but I'm guessing they'll give you extra time.