This plugin allows you to add a custom "Join our Discord" message in the players inventory using an effect.
Download both the .zip (resource pack) and .jar (plugin) file. Put the .jar file into your server plugins folder, and setup the .zip file as your server resource pack with this tutorial: https://www.youtube.com/watch?v=qCVPgn3IppU. I will be updating this plugin asap, so make sure to check in for updates!