Setting custom agent states

How to use api v2 to set CUSTOM agent states please ?

https://{server}/ns-api/v2/domains/{domain}/callqueues/all/agents/{callqueue-agent-id}/{status}
Sets an agent to Online/Offline/Lunch/Break/Meeting/Other ... and works as expected.

However, if I specify a custom state, the response is....
{
"code": 400,
"message": "Bad request. Missing parameter 'device-sip-registration-state'."
}

Someone in slack forums suggested "custom states can't be referred to by name". If this is true, how are they referred to and where is the documentation ?