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.
UNDOCUMENTEDGETGetTrustScore

Returns the Trust Score for the provided account in the specified appid.

https://api.steampowered.com/ITrustService/GetTrustScore/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint64 Yes Steam ID of user for which Trust is being requested.
uint32 Yes App ID for which Trust is being requested.