It is possible to create a quick login for your helpdesk via Facebook To do so, you need to follow these steps:
Visit [https://developers.facebook.com/apps/] and click on "Add a new App"
After that enter the app settings
Navigate to "Settings" and fill in this infromation
Navigate to app review and Check "Make [appname] public?"
Navigate to "Admin -> Security -> Third Party Applications" and enter the App ID and the App Secret. You can find this Key in the Dashbard of your Facebok app
Now you can link accounts via "Avatar -> Profile -> Link Accounts" or login via Zammad login page.