Honeslty as someone in a similar position the only thing you can really do is just do it. If you feel the anxiety is too much I’d disclose that you have a speech impediment and it may take you a little bit longer to get your responses out. From personal experience I had to learn how to not psych myself. Shifting my mindset from thinking of a speaking task as this big monster to something along the lines “ it’s only 15 minutes no biggie .” Take a deep breath before and go in with a mind of certainty. If you have any speech strategies use those. The best strategy I have found though is not just thinking of the stutter. Good luck! No matter what outcome you’ll still come out with something from this experience.