BE: An app deployed to Azure can take advantage of Managed identities for Azure resources, which allows the app to authenticate with Azure Key Vault using Azure AD authentication without credentials (Application ID and Password/Client Secret) stored in the app.
Select Add Access Policy.
Open Secret permissions and provide the app with Get and List permissions.
Select Select principal and select the registered app by name. Select the Select button.
Select OK.
Select Save.
Deploy the app.
References:
https://docs.microsoft.com/en-us/aspnet/core/security/key-vault-configuration
https://docs.microsoft.com/en-us/azure/key-vault/general/tutorial-net-virtual-machine