Zoho CRM client script - Showconfirmation with checkbox

Zoho CRM client script - Showconfirmation with checkbox

Dear sir/madam, 

I am struggling with the following: 
I have made a client script for a Field event. 
It is a very simple function, but if I reject the message the checkbox is still checked. 

I am using the following function:

ZDK.Client.showConfirmation('Bevestiging mailen?', 'Ja', 'Nee');

How could I fix this?

With kind regards, 
Matthijs