Is there a REST API method in Camunda to throw boundary error event to a running process instance?
I have searched a lot but can not find.
You cannot throw errors from the outside - they always happen from within the process model (e.g. Service Tasks or Throwing Error Events).
If you want to influence process flow from the outside use a message - there you will find the proper API method in the docs. Consider is as "error message" :-)
Cheers Bernd