FreeScout can be integrated basically with anything via Make Integration (Formerly "Integromat")

To use FreeScout with Make you will need to install API & Webhooks Module

FreeScout integration looks like this in Make:

Mailbox ID you can find in the URL of the mailbox in FreeScout, for example: https://demo.freescout.net/mailbox/settings/2, where 2 - is the mailbox ID.