How do you use System-assigned managed identity? ManagedIdentityCredential: The managed identity endpoint is indicating there's no available identity

Question

I a resource Azure Media Service and an Azure Web app service.

What I am trying to do is turn the web app's identity into a system assigned identity. I get this error ManagedIdentityCredential: The managed identity endpoint is indicating there's no available identity. Message: undefined Status code: 400

const credential = new DefaultAzureCredential({
    managedIdentityClientId: "0000000-0000-0000-0000-000000000000"
});

• I am using the object ID of the web app.

• I then go to the resource media service (Access Control (IAM)) and turn on these resources: [Media Services Account Administrator, Media Services Live Events Administrator, Media Services Operator]

In my mind I am thinking the accessing resource (web app) Managed Identity would be the "source" and the media service would be the "target. It just gives that error.

I even tried from the web app to use the Azure role assignments Add role assignment (preview).

But what's odd about that is you can't target the resource but only a subscription, resource group and some specific things.

To note, when using the user based assigned identity that works and is probably the preferable way of doing it anyway.