Hi Alberto,
It is not possible to call a javascript function on clicking the icon that you have created by customizing the label, however you can use OnClientNodeClick event to call javascript function on node click.
For further details about client side events please refer to the following online document.
http://help.syncfusion.com/UG/User%20Interface/ASP.NET/Diagram/Documents/4532clientsideevents.htmRegards,
Dinesh