Hi Wilson,
The ExcelMarker sample works with left mouse button and not with right mouse button. The mouse button for dragging can be changed in the MouseDown() method of the ExcelMarkerMouseController class which could be found in the \Syncfusion\Essential Suite\Grid\Samples\In Depth\ExcelSelectionMarker sample.
Also please refer
this sample. Let us know if you have any further queries.
Regards,
Calvin.