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.
GETAuthenticateUserTicket
https://api.steampowered.com/ISteamUserAuth/AuthenticateUserTicket/v1/
NameValueTypeRequiredDescription
click to setstring Yes access key
uint32 Yes appid of game
string Yes Ticket from GetAuthSessionTicket.