I found some info about BridgeAttendedTransfer event in Asterisk 13 wiki:
Added new events for externally initiated transfers. The event BridgeBlindTransfer is now raised when a channel initiates a blind transfer of a bridge in the ARI controlled application to the dialplan; the BridgeAttendedTransfer event is raised when a channel initiates an attended transfer of a bridge in the ARI controlled application to the dialplan.
If this event is ARI controlled then How to trigger it in ARI?
reference, Home - Asterisk Documentation