Create a tracked iMessage opt-in link

Generates a per-campaign link that opens Messages on this sender with body prefilled. A thread the contact opens skips the pacing and the first-message content rule that apply when the sender writes first, and it is the only way in for senders without the add-on for new contacts (their sends to anyone else fail with recipient_must_message_first).

Each link carries a unique code in place of the [opt-in-code] placeholder; when the contact sends it, the resulting message.received webhook (and the stored inbox message's metadata) has optIn: true and your parameters under optInParameters, so you can attribute the conversation to the campaign or lead that produced it.

For an untracked link, use the sender's optInLink instead.