On Microsoft Windows, you can save your Zwift credentials (your username and password) into the "Internet Settings" keystore, but on Mac, things are different. 

Zwift on Mac, by-default, does not let you save your credentials anywhere that it can use when you launch the app, but David A. Desrosiers (aka "setuid") has written a detailed HOWTO that uses Automator on Mac to do just that!

HOWTO: Fully automated Zwift login on Mac OS X

Not only does it auto-log into Zwift for you with a double-click of the icon, but it also securely stores the password in the Mac OS X "Keychain", an encrypted key store that will protect those credentials for you! 

If you want to change the icon of the Automator script after you create it for Zwift, he's created a useful YouTube video that describes that process too!