Firstly, ensure that you have granted your 'Training' consent on Garmin. To do this, log in to your Garmin account and go to this link.
Make sure you have accepted the 'Training' consent':
You can also find this consent directly from the Garmin Connect app, in the menu 'Settings' > 'Connected Apps' > 'Decathlon.'
Once the consent is accepted, you can return to the Decathlon Pacer app and click on the 'Export' button.
Your session will be exported to Garmin and visible in the Garmin calendar here. Remember to sync your Garmin watch and Garmin Connect app for the transfer to occur on your watch!
Please note that, currently, this feature is only compatible with Garmin watches.