According to the documentation of the event beforesubmit in Ext.form.Panel , I should be able to modify the values submitted, but I can't figure out how.

Implementations may adjust submitted form values or options prior to execution.
I've tried modifying the variable values, but this has no effect.

Any help is highly appreciated.

Note: I'm basically trying to send a JSON string, so I need to encode the values using JSON.stringify.