Validation Message in Event Switcher

I have made a simple validation within the Event Switch code.
If the amount is less than 5 I want to display an error message.
I have coded:

if ( $amount < 5 ) {
$form->validation_errors['Bedrag'] = "Foutief Bedrag";
return "fail";
}

The validation is executed correct because of the status 'fail' and the executing of the 'on fail' functions (Event Loop - Load), but the error message is not displayed.
I think the code $form->validation_errors['Bedrag'] = "Foutief Bedrag"; will not work in Chronoforms V5.
Helas I can't find another method.

Thanks for helping.

Thank you Seagul.
It works!
Sometimes you don't see the obvious solution.

In the meantime I found out an other soulution:

$app = \JFactory::getApplication();
$app->enqueueMessage('Foutief Bedrag. SVP minimaal 5 euro doneren.');