Trigger a preconfigured event template for one or more recipients. Identify the template with event and pass each recipient's values in template_variables.

Modes — pick one:

• Batch: set top-level sender and event; omit them on each recipient.
• Mixed: omit the top-level values and set sender and event on every recipient.

Returns a batch task_id plus per-message identifiers; partial failures are reported inline.

Authentication: stream token in the Authorization header (see Authentication in the API Overview).