I used tcpdump with a dummy Plex server that listened to incoming requests [1]. Because those request are automatically generated when a user opens up the Plex app. And I than proceeded to send a WoL request.

A reverse proxy would of probably worked too, but I didn't want to be limited by the 100Mbps network interface of my Pi 3B.

1. https://gist.github.com/alex3305/8cc73ddd2c8ca6328f20235480a...