Live-streaming in ZOOM Pro account is limited with three predefined social networks and one hard written rtmps url per account.
So at the end of the day a User Pro has a choice among available hard coded social networks or ONE and single rtmps option
If the ZOOM Pro user want to stream to another Custom Live Streaming Service he must edit his web account and override the rtmps url.
Have you ever considered including multiple rtmps url in Zoom Pro Account Web Form which would generate a choice of multiple RTMPS options in Live on Custom Live Streaming Service selector?