That depends on the specific duties and applicable practice rules, as well as effective immigration authorization. Do not assume TN permits regulated services before the required licence is in place. Obtain a clear assessment of any proposed initial tasks before performing them.
Align the first task with both permissions
Ask the employer to identify what the applicant would do on day one. Compare that with the regulator’s requirements and the immigration record. If a staged start is proposed, its tasks must be real and lawfully permitted, not a temporary label for restricted work.
Begin the licensure file as soon as the destination state is settled, because verification from an institution or a home-jurisdiction regulator involves third parties. Give the employer a specific deadline tied to the intended filing or travel date, and hold that date rather than proceeding with a letter you expect to correct later. Ask what changed on the effective date: only a human-resources band, or also the employer, core functions, worksite, compensation, ownership or duration. Make a dated sequence covering the offer, planned admission or I-129 route, authorization point, travel, and any proposed first day of work. A Canadian citizen may apply at a port of entry or an employer may file Form I-129, but the parties should not promise work before authorization.