I had an IELTS English exam once and I just went straight up with the examiner that I could have a stutter during the exam every now and then. If you tell them they know and they can take it into account for their grading. If you don’t tell them they can start thinking that you are looking for words or something else. So I would tell them in advance also to make it easier for yourself. Stuttering is something you can’t do anything about, I know it’s frustrating but we can’t help it!! Please keep that in mind as well! Good luck with your exam, break a leg!! Edit: please let me know how it went Edit 2: just came up with this, you can get bonus points if you can tell it in the language of your exam :)