Hi @Mario Bondici
I'm able to setup MySQL in-app backup successfully by using the connectionString from file mentioned above. Screenshot A below shows how i acquired the connectionString (using Console under Development Tools) which i have later added to the Connection strings section under Application Settings (Screenshot B) which then made the MySQL in-app as an available option on Backups blade.
Refer - https://github.com/MicrosoftDocs/azure-docs/issues/8454
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