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.
PUBLISHERPOSTGetCoPlayStatus

Get coplay status for two accounts

https://partner.steam-api.com/IOnlinePlayService/GetCoPlayStatus/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 Yes Steam ID 1 of request
uint64 Yes Steam ID 2 of request
uint32 Yes App ID of request
uint32 Yes unix time range begin to check
uint32 Yes unix time range end to check