First you need to have your Zwift account connected.
You can do that from the web version of the platform from the "User Settings" menu. The full guide to that is here.
Once you are connected, when you open a workout that is compatible with Zwift (is power and time based) you will see the "Send to Zwift" button. You can do this from the new beta view of the workout on web and mobile, or from the legacy view on web. You can send 7 days in advance, or individual workouts one at a time.
Sending 7 Days in Advance
From Web App
From the More Tools - Sync to Device dropdown
From Mobile App
From the profile on the mobile app.
Sending Individual Workouts
From the new Beta view of a workout.
From the legacy calendar view.
