Raise it as soon as the offer includes equity, before the support letter and supporting evidence are drafted. The description of the employment relationship in those materials should match the actual arrangement, and that is difficult to correct after documents have been submitted or presented.
Align the equity timeline with the application timeline
Note three dates side by side: when the option vests, when the applicant would exercise, and when TN admission or a TN filing is planned. If a vesting date falls inside the intended TN period, decide in advance what happens then rather than at the moment it arrives. Where the ownership picture would change mid-period, ask counsel what needs to be disclosed and when.
A financing round is a commercial event on a commercial calendar, and it will not wait for an immigration one. When a company raises capital or is bought, options are frequently accelerated, repriced or converted, so an ownership picture reviewed in the spring can be superseded within days of a closing. Ask whether any round, sale or reorganisation is expected during the intended TN period, and treat the answer as part of the timeline rather than as company news.
Hypothetical example: a food technologist admitted for prearranged work at a produce-processing firm learns that a planned buyout would convert their options into a controlling personal block; the choice to make before the closing is which of the two to keep, since controlling the company being served is treated as the self-employment this category does not permit.