I tell HR ahead of time (or whoever sent you the email or phone call to invite you to the interview). I’ll copy and paste mine below from 2 months ago “ Hi Tori, Thank you so much for an invitation for an interview. Just wanted to let you know early, that I have a speech disorder called stuttering that tends to only occur in an interview context. So if you could please let Dwight and Liam know ahead of time that my stutter will come off quite moderate during the interview but is almost non-existent when I start working, which my referees can vouch for. Thank you again.”