Create a subscription (URL and chains), optionally with initial addresses.

chains is required and has at least one key; without it (or with an empty object) the call is 422 chains_required and data.available lists the chains open for push; there is no default chain, nothing is created and nothing is billed. A chain whose value is {} uses the chain default confirmations. The subscription is online at once; every chain starts after the chain head at creation (start_block is stamped by the platform, usually within a second, and is null until then); events from before creation are not delivered and replay cannot bring them (there were no matches yet), use the Data API by address. There is no verification or test message: the first message carries on-chain events. The response carries the signing secret, the only time it is shown. The calling key becomes the billed key; a key whose CU cap is used up can still create it; delivery pauses as soon as the key cu_cap or the account balance is exhausted and resumes without loss after a top-up or a cap increase; the condition field reflects settled usage and can lag up to one settlement period (about one hour). Optional addresses uses the same normalization, set semantics and admission as addresses/add (1 to 10,000 items; the creation body still must fit in 64 KiB). Creation and initial addresses commit together, or nothing is created. When omitted the subscription starts with no addresses; add more with the address endpoint. Each subscription holds at most 1,000,000 distinct addresses.