Azure – ARM Template – Deployment slot keyvault permissions

Trying to assign permissions for the app service deployment slot to keyvault and having a hell of a time getting it

{
                        "tenantId": "[subscription().tenantId]",
                        "objectId": "[resourceId('Microsoft.Web/sites/slots', parameters('azureAppService').webSiteName, 'DEV').identity.principalId]",
                        "permissions": {
                            "secrets": [
                                "Get"
                            ]
                        }
                    }

Not sure what I’m doing wrong here, the template validation goes through, but upon deployment but I get an error. How do I specify the resource ID for the deployment slot?

Here is the error

{
"status": "Failed",
"error": {
    "code": "InvalidTemplate",
    "message": "Unable to process template language expressions for resource '/subscriptions/---/resourceGroups/Test/providers/Microsoft.KeyVault/vaults/KEYVAULT-TEST' at line '447' and column '9'. 'The language expression property 'identity' can't be evaluated.'",
    "additionalInfo": [
        {
            "type": "TemplateViolation",
            "info": {
                "lineNumber": 447,
                "linePosition": 9,
                "path": ""
            }
        }
    ]
}

}