Haia, I wanted to share the script that we're using at our organization currently which utilizes Zoho Workspace organizations (supports free tier), it's a custom OAuth using Zoho Mail API. It works by simply comparing if Zoho email matches one in the help desk admin, each account on Zoho can have only 1 unique main email address which prohibits any bypasses.
Feel free to use, or contribute. The script does not require any major changes to any Hesk files.
https://github.com/CryBlanka/hesk-zoho-oauth