Adding a room to a community(using api)

We are trying to add some rooms to communities using events endpoint like this:
_http://${MatrixHost}/matrix/client/r0/rooms/${roomId}/state/m.room.related_groups?user_id=${masqId}

...and sending what it seems adding a room from the community page manually sent as event content:
{"groups": ["+agroup:adomain.net"]}

The event m.room.related_groups is registered but after clicking communities icons to filter rooms, only the ones added manually are visible.
Not sure it is just missing api or hidden since I cannot find any api documentation on it.

Start by examining the events endpoint for m.room.related_groups and compare its handling with the event content sent when adding a room manually. Check the existing API documentation for this endpoint and event. Done means the supported API behavior is established and the discrepancy is either documented or reproducibly corrected.