public string GetObjectIDField()

{
  //Url examples for (federated) feature services
  //(by "ref" online):
  var url =
      @"https://sampleserver6.arcgisonline.com/arcgis/rest/services/SF311/FeatureServer";
  //(federated by ref on portal)
  //https://portal.example.com/server/rest/services/FeatureServiceName/FeatureServer");
  //(federated by value - Hosted - on portal)
  //https://portal.example.com/server/rest/services/Hosted/FeatureServiceName/FeatureServer");

  await QueuedTask.Run(() =>
  {
    var uri = new Uri(url, UriKind.Absolute);
    using (var fs_db = new ArcGIS.Core.Data.Geodatabase(new ServiceConnectionProperties(uri)))
    {
      //Assuming that id 0 is a FeatureClass for the given service
      FeatureClassDefinition featureClassDefinition =
          fs_db.GetDefinition<FeatureClassDefinition>("0");
      string shapeField = featureClassDefinition.GetShapeField();
      IReadOnlyList<Field> fields = featureClassDefinition.GetFields();

      //Assuming that id 1 is a Table for the given service
      TableDefinition tableDefinition =
            fs_db.GetDefinition<TableDefinition>("1");
      string objectIDField = tableDefinition.GetObjectIDField();
    }
  });
}