This page is just a reference of all the known Steam APIs, I do not know how they work. Please do not email me with questions.
POSTPlayerStats
https://api.steampowered.com/ISteamBroadcast/PlayerStats/v1/
GETViewerHeartbeat
https://api.steampowered.com/ISteamBroadcast/ViewerHeartbeat/v1/
NameValueTypeRequiredDescription
uint64 Yes Steam ID of the broadcaster
uint64 Yes Broadcast Session ID
uint64 Yes Viewer token
int32 Novideo stream representation watching