FrameworkApplication.SetCurrentToolAsync("esri_mapping_selectByRectangleTool");

          // or use ICommand.Execute
          ICommand cmd = FrameworkApplication.GetPlugInWrapper("esri_mapping_selectByRectangleTool") as ICommand;
          if ((cmd != null) && cmd.CanExecute(null))
            cmd.Execute(null);

 FrameworkApplication.ActivateTab("esri_mapping_insertTab");

来自：https://github.com/Esri/arcgis-pro-sdk/blob/master/Examples/Framework/ProSnippets.cs