I don’t really use workouts much but agree the search facility could be better.
For now the best help I can give you is if you know the full name and know it is still in Zwift [2500+ workouts are in Whats on Zwift but are not in game] then you can search All Workouts alphabetically in Zwift Companion App or in game - Not ideal but possibly better than nothing.
Have a read through the first post of this thread as it may explain how you can transfer/copy workouts from What’s on Zwift using PC or Mac.