FM Javascript question

I am trying to change the value of a hidden form field using javascript. I set up the field with a nickname and I’m using a condition to hide it.

The value needs to come from a php variable on the page that the form is displayed on.
php:

$tempDate = $event->get_timespan_html();
$tempDate = print_r($tempDate, true);

Since I can’t seem use any php variables in my custom fm form template or in a FM note field for javascript, I’m trying to use some javascript before the form template loads:
javascript:

window.onload=function(){
var eDate = fm_get_form_item('eventdate');
eDate.value = <?php echo $tempDate ?>;
};

This doesn’t work. Although I do get the correct value in the script, the “eventdate” form field does not populate. Upon submitting the form, the “eventdate” form field remains empty. And of course my e-mail template does not show the eventdate value.

Any ideas on how to get this to work?

http://ww.wp.xz.cn/plugins/wordpress-form-manager/