According to https://docs.vrchat.com/docs/vrc_syncvideoplayer , VRC_SyncVideoPlayer has AddURL PlayIndex and Clear RPCs. But these don't appear in "Events from Scene" menu of VRC_Trigger.
1. You check VRC_Trigger advanced mode checkbox to enable
2. And you directly add SendRPC action from "Basic Events" menu.
3. And you set up Receivers object properly.
Then, These RPCs appear in Method drop down list.