public class ReportCustomTemplateDefinition 

//Note: Run within QueuedTask.Run()
await QueuedTask.Run(() =>
{          
  // get the first map in the project
  MapProjectItem item = Project.Current.GetItems<MapProjectItem>().FirstOrDefault(n => n.Name == "Cities");
  if (item == null)
    return;

  Map map = item.GetMap();
  if (map == null)
    return;

  Layer cityLayer = map.FindLayers("U.S. Cities")[0];
  if (cityLayer == null)
    return;

  // load template information
  var templatePath = System.IO.Path.Combine($@"{Project.Current.HomeFolderPath}\TemplateSimpleReportWithTokens.rptt");
  ReportCustomTemplateDefinition templateDef = ReportTemplateManager.GetCustomTemplateDefinition(templatePath);
  if (templateDef == null || templateDef.ReportTemplateDataSourceItems?.Count() < 1)
    return;

  // get template info of first data source (parent) and set its data source
  ReportCustomTemplateDataSourceInfo parentDataSourceInfo = templateDef.ReportTemplateDataSourceItems.ElementAt(0);

  parentDataSourceInfo.SetReportDataSource(cityLayer);

  TemplateTokenFieldInfoValues templateTokenFieldInfoValues = parentDataSourceInfo.TokenInfoValues;

  parentDataSourceInfo.DefinitionQuery = "STATE_NAME = 'Massachusetts'";
  templateTokenFieldInfoValues["[text-field-value-0]"] = "CITY_NAME";
  templateTokenFieldInfoValues["[text-field-name-0]"] = "City";
  templateTokenFieldInfoValues["[numeric-field-value-0]"] = "POP1990";
  templateTokenFieldInfoValues["[numeric-field-name-0]"] = "Population";

  if (templateDef.CanCreateReport())
  {
    // create the report
    report = ReportFactory.Instance.CreateReport("RelateSiblingsFromCode", templateDef);
  }
});
//UI thread
if (report != null)
{
  //open report
  await FrameworkApplication.Panes.CreateReportPaneAsync(report);
}