Create a playbook or campaign draft

Prefer mode:"always_on" (a playbook) with no recipients: limits default from the gift (budgetCents 100,000,000, perPersonCents the gift and shipping, maxSends 2,000). Batch and single campaigns also need budgetCents, perPersonCents and maxSends (at most 500 people). Saves a draft; a signed-in person turns it on at https://gtmswag.com/campaigns/{id}?review=1. Keys cannot set status:"active" (403 HUMAN_APPROVAL_REQUIRED) or autoApprove. Idempotent with idempotencyKey: the same key and body return the same campaign (201); the same key with a different body returns 409 IDEMPOTENCY_CONFLICT.