Ah, I think you are referring to starting a workout on your Apple Watch independently. This is possible 
I don’t honestly know which the best option is, but essentially you can:
-
Record the workout independently of the Freeletics App on your Apple Watch. Essentially you can start to record data like this on your Watch by starting a workout ( I would suggest HIT but it’s not always the case) in the Workout app-this way all your HR, estimated calorie expenditure etc is sent to Activity and the Health app. I would make sure you disconnect any connection between Apple Health and the Freeletics app in your app settings-otherwise you will get data from both the workout recorded on your Watch, and the data synced to Health from the Freeletics app.
-
Just allow the connection between the Freeletics app to Apple Health, and Apple will estimate calorie data based on the workout in your Health app.
The below response provides maybe a bit more data 
And, as pointed out by @Sushant in the same conversation, wearables are notoriously bad at estimating calorie expenditure
