Where to push the feed for a stream created with a push input type (RTMP_PUSH, SRT_PUSH).

Requires streams:write rather than streams:read because the response includes stream_key, which is a credential.

Provisioning happens after stream creation, so the fields are null until it completes. Poll this endpoint, or wait for the stream's status to leave pending. Streams using a pull input type return pull_source_url and no push credentials.