You can connect MyFitnessPal through the Apple Health app. 

1. Open the Health app.
2. Tap on your profile icon in the upper right corner.
3. Scroll to Privacy and tap on Apps.
4. Tap on MyFitnessPal.
5. Select either Turn On All or specifically select what data you want shared.
6. Then, repeat the same steps for The Fitness App if you have not already done so.