Activates a subscription that is not active yet. Send expectedRevision (the subscription's revision, 0 before one exists) and a requestId of your own; the same requestId never applies twice.
Who may call it: A signed-in buyer (X-Storefront-Buyer); without one the answer is 401 UNAUTHORIZED.
Responses
default
The request was refused or could not be completed; the status says why (400 validation, 401 sign-in needed, 403 not allowed, 404 unknown, 409 stale revision or version, 422 commerce rule, 429 too many, 503 unavailable).