Javascript custom validation

I am building a function on Javascript for do some validations own and I am following the FAQ "Validating fields" in the point "Custom validation function" and it doesn't work.

This code doesn't work:[pre]jQuery.fn.form.settings.rules.customfn = function(value, param) {
return false;
};[/pre]
It work with this code:[pre]function customfn() {
return false;
};[/pre]
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In the event "Call function" en "Action Parameter" I write "customfn".
If I write "customfn:error-message" doesn't work too.[br][br]How mush I perform the call to the function and place the error message? [br][br]Thanks.

Someone can I help me?.
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Thanks.