Hi
To record and save a workout as Strength Training on Apple Watch:
Record your workout using the built-in Workout app, choosing "Other" as the activity type > when you have finished your workout, swipe right and tap End > scroll down and tap on Name Workout > choose Strength Training.
The workouts that you do most often will be offered as Quick Start options within the Workout app (including previous "Other" workouts with their saved labels pre-selected).
More information: