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.
GETGetBroadcasterInfo
https://api.steampowered.com/IDOTA2StreamSystem_570/GetBroadcasterInfo/v1/
NameValueTypeRequiredDescription
uint64 Yes 64-bit Steam ID of the broadcaster
uint32 NoLeagueID to use if we aren't in a lobby
UNDOCUMENTEDListOfUsersSpectating
https://api.steampowered.com/IDOTA2StreamSystem_570/ListOfUsersSpectating/v1/