See solution below.
Explanation:
With the storage account ready, you can configure backs up in the web app or App Service.
* Open the App Service az400-11566895-main, which you want to protect, in the Azure Portal and browse to Settings > Backups. Click Configure and a Backup Configuration blade should appear.
* Select the storage account.
* Click + to create a private container. You could name this container after the web app or App Service.
* Select the container.
* If you want to schedule backups, then set Scheduled Backup to On and configure a schedule: every five hours
* Select your retention. Note that 0 means never delete backups.
* Decide if at least one backup should always be retained.
* Choose if any connected databases should be included in the web app backup.
* Click Save to finalize the backup configuration.

Reference:
https://petri.com/backing-azure-app-service