Adds recipients to a sequence and, unless they are kept as drafts, starts sending to them. You need the sequence activation feature, otherwise the response is 403 with an empty body.

The response is 200 even when some or all recipients were not added. Check each entry's status: success (added), active (already active in another sequence and this sequence prevents double booking), duplicated (already in this sequence), unsubscribed or domain-unsubscribed, or error with the reasons in errors.

To keep a recipient in the draft without sending, set scheduledAt: false on that recipient. A top-level scheduledAt: false is treated as "now" and sends right away.

If the sequence uses variables in its subject or body, each recipient must have a non-empty value for them, otherwise it gets an error entry. Set allowMissingVariables to skip that check. An email or Email variable, when given, must be a valid email address.

A request with an empty recipients array returns {"recipients": []} without checking the sequence. More than 5,000 recipients in one request returns 422.