Not sure I understand correctly. What kind of fitness app? The best way is recording your workouts with your Garmin device. If you have some data from a 3rd party app that you wish to import, you can do it on the page https://connect.garmin.com/modern/import-data
There are also many 3rd party apps that can be linked to Garmin Connect though their API. Some of them are listed for example on the page Link Garmin Connect Activities With Third Party Fitness Websites, but in fact there are many more of them. Check out with the author of your app, to see whether it can be linked directly to Garmin Connect.
