Declare as DBusActivatable and install D-Bus service file
As Elisa runs as single instance (due to KDBusService(KDBusService::Unique)) it can support DBus activation (instead of having another Elisa process created which then internally by KDBusService tries to see if Elisa already runs and, if so, forwards the arguments and kills itself again.
See the spec in https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#dbus
KDBusService provides the needed code support already out-of-the-box. Launch code on KDE side is currently getting support for this as well, see frameworks/kio!363 (merged) & frameworks/kglobalaccel!11 (merged).