How to access an Akka actor "from the outside" of the app? via its pid or equivalent?

How to access an Akka actor "from the outside" of the app? via its pid or
equivalent?

Context: a stateless webservice (running on Glassfish) which uses Akka
actors to run jobs. Results from this service are stored in a database.
The webservice client is used to launch jobs by the web service. This part
works fine. I need now that in a different session, the web client can
send an instruction for a job (my Akka app) to interrupt in an orderly
fashion. This would be done best by sending a message to the top actor,
which would then dispatch further messages to its actors.
The problem is, I don't know how to send a message "from the outside" to
an actor of an akka app which is already running?
Any pointer would be appreciated!