To use the custom validation in Nintex, in the control settings, provide a method name for your custom validation for that specific control:
- normally, you're supposed to be able to use arguments.Value but it seemed to return blank for both checked and unchecked.
- When using jQuery in Nintex Forms, use NWF$() instead of the standard $(), as a result of compatibility mode.
- You can get the client ID (the large random-looking auto-generated ID provided by SharePoint) by specifying an name for Nintex Forms to store the client ID in, as part of Control Settings.
And that's all you need to do to get custom validation working for Nintex Forms :)