When to inform potential employer about need for TN status?

I'm a medical professional qualified under NAFTA, and I was a student (F-1 OPT) turned TN status with my current company so I didn't have to go into a new job with TN.
My question is, when should I tell my potential employer about my status? For us, the hiring process usually has no recruiter involved before you're considered. I apply online, I don't check the box for sponsorship/visa; then they set up and interview, usually either with the chief of the department, and I've also had interview where I meet with the director, coordinator and then the team I'll be working with. Then HR/recruiter will contact me regarding further steps (eg. references, salary negotiation etc.)
When in this process should I tell them? I was thinking either in my resume, during the interview with the director or in the thank you email. I also don't want to feel like I'm withholding information. :/
Thanks a lot!
There is no correct answer. You know your industry the best.
For me, I put "Eligible for TN status" on my resume and cover letter when applying.