Once created you can select the workout text, copy and paste into an open empty Zwift workout file (*.zwo) on your desktop computer. On my device it is stored in a sub-directory off my document file /Zwift/Workouts/123456 When Zwift is then run on that device it should pick up the workout file and put it in the data for you on its server. It should then appear under custom workouts. You can then select the workout on any device.
I don't think the implementation of running on Zwift to be complete and in my opinion it should be considered in beta though it remains incomplete after many years. It uses best mile pace for running workouts with a nominal power of 1.0 at that pace. You have to enter that mile pace via a typically terrible gaming data entry interface. In my case a workout with a phase with a power of 1.0 does not seem to accurately represent the value I entered in user settings. I recommend generating a test workout containing a phase of power 1.0 and confirming this results in your mile pace by inspecting the workout in Zwift. If it doesn't you may have to spoof the value entered in your user settings on Zwift to ensure the test workout does indeed produce your mile pace. It is painful but once done seems set longterm. I would leave the test workout on Zwift and just take a look at that single phase of power 1.0 and ensure the mile pace stays correct. Your race pace over a mile based on the 10k time you entered above is 0'' / mile OR 0'' / km