This is assuming that you are using webOS OSE, so you can build your system using bitbake.. if you install a file that looks a little something like this into /etc/init:
description "starts my web service"
start on (settingsservice-ready) or unified_service_server_restart
stop on started start_update
exec run-js-service -u -g -k -w -n /usr/palm/srevices/(yourservicedirectory)
then that should get you launched at startup.
I'd have to look up the specifics for doing it via activitymanager, which is the way i would do it if I didn't want to modify the startup files