WaitForCompletion.cs

 {
    "name": "Wait For Completion",
    "type": "WebActivity",
    "dependsOn": [
        {
            "activity": "Start Long Running Task",
            "dependencyConditions": [
                "Succeeded"
            ]
        }
    ],
    "typeProperties": {
        "url": {
            "value": "@activity('Start Long Running Task').output.statusQueryGetUri",
            "type": "Expression"
        },
        "method": "GET"
    }
}