Within Zwifts workout section there is a part called custom workouts, you can click the plus sign and create your own workout using a drag and drop system. It doesn\’t work the best but it still does the job.
You can click the text boxes to change the amount of watts you are doing and the amount of time you are doing it for. You can also add an interval set, a message during the workout, free ride time, and cadence drills. All you have to do is drag the blocks around to build your custom workout. It is fairly simple. Here is a quick video clip of me creating a custom workout. You can also share workouts with other riders by copying the file and pasting it in other peoples workout folders. If you are on Mac then you just drag and drop the file. This is the link to the video
There is another option on the bottom of the screen that allows you to turn erg mode on and off. You can also select tags to help you sort through your workouts easily. You can also choose to name your workout whatever you want and even add a description of what this workout is and anything else you want to write about it so that you remember.