Apply only when the letter is final. A port-of-entry application can produce a same-day decision, so the sequence is letter first, application second, start date third, not the reverse.
Do not present a draft to a port of entry
Hypothetical example: a U.S. company wants a Mission forester on site for the start of a planning season and offers to send a corrected letter after she is admitted. That inverts the order.
The officer decides on the letter presented that day; a later correction does not repair an admission granted on incomplete facts, and a refusal is recorded. The safer sequence is to settle the letter, then apply at a port of entry or preclearance location with a few days' margin before the intended start, then travel. Admission can be granted for up to three years and extended by a new application or petition.
If the employer cannot finalize the letter in time, moving the start date is cheaper than an application that fails on a detail everyone knew was still open. Hypothetical example: An agricultural scientist has a seasonal research start date, but the employer's credential copy is missing and the duties letter names a different project. The first review should set a document-completion date ahead of the proposed travel, then test whether the final package tells one consistent story.
The letter, passport and credential proof are the records that settle readiness; a planned correction after admission does not answer questions asked at application. If the employer needs certainty earlier, it can consider the petition process instead of relying on last-minute travel. Do not begin services until the authorized period and employer arrangement cover the actual work.