@KeyVault settings not being processed

I would like to use the feature of combining KeyVault and managed identities to secure certain settings.

I am using AF 2, have deployed using VS 2017 (15.9.5) Azure Functions and WebTools 15.9.02046.0 and have deployed to West Europe.

I am trying to use the functionality from this article https://azure.microsoft.com/en-us/blog/simplifying-security-for-serverless-and-web-apps-with-azure-functions-and-app-service/

I have created an MSI and connected it to my KeyVault.

The setting in the function app is under App Settings and the values are:
"DatabasePassword" : "@Microsoft.KeyVault(SecretUri=https://MyKeyVaultName.vault.azure.net/secrets/[secretuser]/[identifier])"

When I run the code I do not get the value from the KeyVault, I get the setting value.