I’m trying to get a OATH token for the ADF managed identity to query Azure Storage APIs, this is the Web Activity I’m using

{
    "name": "Get Bearer Token",
    "type": "WebActivity",
    "dependsOn": [],
    "policy": {
        "timeout": "0.1:00:00",
        "retry": 1,
        "retryIntervalInSeconds": 30,
        "secureOutput": true,
        "secureInput": false
    },
    "userProperties": [],
    "typeProperties": {
        "method": "POST",
        "headers": {
            "Content-Type": "application/x-www-form-urlencoded"
        },
        "url": {
            "value": "https://login.microsoftonline.com/@{pipeline().globalParameters.TenantId}/oauth2/v2.0/token",
            "type": "Expression"
        },
        "body": {
            "value": "concat(n    'grant_type=client_credentials'n    ,'&scope=https://storage.azure.com/.default'n    ,'&client_id='@{variables('adf_principal_id')}'n)",
            "type": "Expression"
        }
    }
}

The body is this – so it’s more clear

concat(
    'grant_type=client_credentials'
    ,'&scope=https://storage.azure.com/.default'
    ,'&client_id='@{variables('adf_principal_id')}'
)

I’m getting this error

The request body must contain the following parameter: ‘grant_type’

What is the grant_type for a managed identity?
How do I fix this?