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.
UNDOCUMENTEDPOSTGetVideoBookmarks

Returns the video bookmarks locations for the specific videos. Includes playback settings per video

https://api.steampowered.com/IVideoService/GetVideoBookmarks/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
uint32[] NoList of App IDs to grab bookmarks for. Can be empty if using updated_since.
uint32 NoOnly return results after time. Min value is 1. (seconds since epoch January 1st, 1970 Unix Time)
UNDOCUMENTEDPOSTSetVideoBookmark

Bookmarks the locations in the video the user has reached. As as record playback settings per video. Fire and forget.

https://api.steampowered.com/IVideoService/SetVideoBookmark/v1/
NameValueTypeRequiredDescription
click to setstring Yes Access key
VideoBookmark[] Nolist of bookmarks we want to store.
↳ uint32 No
↳ uint32 No
↳ uint64 No
↳ uint64 No
↳ uint64 No
↳ uint32 No
↳
bool No
↳
bool No