External Table in ADX

Hi,

I'm trying to create an external table in ADX which uses a Synapse Analytics (SA) database view (called undelivered). The undelivered view itself is query data from a Cosmos analytical store

• I've create a user defined idenity
  • Added the identiy to the ADX cluster, SA and Cosmos
• Updated the ADX database:
  .alter-merge cluster policy managed_identity[
      {
        "ObjectId": "a3d7ddcd-d625-4715-be6f-c099c56e1567",
        "AllowedUsages": "ExternalTable"
      }
  ]
• Created the database users in SA
  

  -- Create a database user for the ADX Managed Identity
  CREATE USER [adx-synapse-identity] FROM EXTERNAL PROVIDER;

  -- Grant read permissions
  ALTER ROLE db_datareader ADD MEMBER [adx-synapse-identity];
  GRANT SELECT ON OBJECT::undelivered TO [adx-synapse-identity];

• From within SA I can "SELECT * FROM undelivered" and the correct information is returned
• But when I come to create the external table in ADX:
  .create-or-alter external table MyExternalTable
  (
      Status: string
  )
  kind=sql 
  table=undelivered
  (
      h@'Server=tcp:synapse-xxxxx.sql.azuresynapse.net,1433;Database="Registration";ManagedIdentityClientId=<key>;Authentication=Active Directory Managed Identity;'
  )
  with (
      managed_identity = "<key>"
  )
  I get the error: Managed Identity 'system' is not allowed by the managed_identity policy for usage: ExternalTable

So even with me specifying the managed identity I want to use it is still trying to use the system one. 

How can I get the external table created with the correct managed identity?
Any questions please just ask
Thanks