Create an API application through Trakt here. The Redirect URI should be your goplaxt.domain + /authorize, so it reads as: https://goplaxt.domain.com/authorize.
Edit the Goplaxt section in saltbox settings.yml: substituting your own ID and secret.
goplaxt:trakt_id:IDHEREtrakt_secret:SECRETHERE
Run the role install command
sbinstallsandbox-goplaxt
Visit the goplaxt site at https://goplaxt.domain.com.
Enter your Plex Username then Authorize, and add the Webhook in Plex Settings.
Make sure under your server Settings > Network that Webhooks is enabled.