Editing the source code of the form definition
The "Edit Source" button in the toolbox opens the "Edit Source" dialog.
Form Builder uses XHTML+XForms as form definition representation. The format must be well-formed XML. The dialog shows a text editor containing that representation.
It is possible to edit the XML representation in the dialog, or to copy/paste it to an external editor for further changes. After saving changes from the dialog, those are immediately taken into account by Form Builder.
NOTE: You have to be cautious when doing so because it is possible to break the form by making changes! We recommend that you learn about XForms before making such changes.