Well you have to tell them because it's not your legal name and they'll need your legal name if you're to be hired there (for payments and taxes). Also because you're not out to references. You're just going to have to be to the point and tell them, but I'd wait until the very, very end of the interview or interview process. Sell yourself first so that they know you're worth it. If you tell them straight up from the beginning, they'll maybe judge you from the start and it'll affect how they legitimately feel about you as a potential employee.
My 2 cents:
To be honest, and I know it sucks, I think for someone in your situation (I'm in the same boat), it's better to avoid this whole pre-process and apply with legal name. You have to watch because legally you haven't changed your name so you're essentially lying in applying and even that alone could go against you. If you don't want to give away your gender, however, it's PERFECTLY acceptable to use initials in applications and resumes.
I decided to just apply for jobs right now in my current legal name and then when I get a job
and sign a contract, that's when I head to HR and say, "Oh by the way..."
It might be something to consider if you don't end up getting this job and apply for another and your name isn't legally changed. Unfortunately, people tend to be more accepting of assuming someone is a butch lesbian than trans. I don't change anything about me going to the interview. I still wear male clothing (although I don't interview in a suit because that's unheard of in my industry), and had my binder and short hair, I was just the old name. However, once you sign a contract, you're often protected by certain employment laws (look them up where you are) which often means they can't fire you if you come out at work, or you at least have a case if they try to. For me, I couldn't change my name because I'm waiting for a permit. Now I have that permit, I'm starting the process next week. If I get this job, hopefully it'll be either in process or already changed. Either way I'll be informing them...as soon as I sign the contract

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