Hi Blanca,
Thank you for contacting Syncfusion Support.
You can achieve this by overriding the RectangleTool class. Please refer to the sample in the following link which illustrates this:
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=RectangleTool620762366.zipNote:
In the above sample, select Rectangle Tool from Tools -> Rectangle Symbol
Please try this and let me know if this helps.
Thanks,
Melba