Asignar un valor de una función Javascript a un campo en Formidable

<script type="text/javascript">
jQuery(document).ready(function($){ //set a random 9-char key
    var currentValue = '';
    var lenght = 9;
    var randomChars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789abcdefghijklmnopqrstuvwxyz0123456789';
    var result = '';
    currentValue = $(field_mediakey).val();
    if ($(field_mediakey).val() == '') {
    for ( var i = 0; i < lenght; i++ ) {
            result += randomChars.charAt(Math.floor(Math.random() * randomChars.length));
        }
        $(field_mediakey).val(result);
    }
    $(field_yzjna).val($(field_mediakey).val());
});
</script>