Hey there,
There is no way to do so at this time.
It might be worth considering using something like TrainingPeaks to get your custom workout uploaded there and then synced to TR using your Activity Sync page, but you still might have to get on your PC to do that.
Or, as @svens mentioned, you could push the custom workout straight to your head unit, do the workout, and then upload it to TR. It won’t have a Workout Level associated with it, but the workout will still be taken into account by AI FTP Detection and RLGL, as stated above.