test-arm.json

{
    "$schema": "https://schema.management.azure.com/schemas/2015-01-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {},
    "variables": {},
    "resources": [],
    "outputs": {
        "test": {
            "type": "string",
            "value": "[reference(ResourceId('Microsoft.Web/sites', 'hello-asl-7cyvgt4ucnwt6'), '2018-02-01', 'Full').identity.principalId]"
        }
    }
}