Especially if the first round screening interview is on the phone, I will tell them something similar to what's been said above, explain what it is and that's it's not the phone line cutting out or something like that. My script generally includes something along the lines of "it's something I've always had, it's something I'm always working on, and while it does pop up from time to time - it hasn't ever stopped me from interacting with people at work or doing what needs to be done." The reaction is always extremely positive, they say that they understand, appreciate the heads up and we move on. This way when you do stutter later during the interview it's much less of a big deal. Practice this straight up disclosure in lower stakes environment, to desensitize yourself from yourself