Inserting Hidden Fields Before A Form Submit()

I am trying to do something, which should be fairly easy. I have a link that is defined as:

<a href="javascript: doTemplates(<?php echo $toggle_templates; ?>);" title="Click to show/hide message templates.">

The php code simply echos either 1 or 0 depending if the templates should be shown. Then I have the following function, doTemplates() defined as:

function doTemplates(input)
{
document.write('<input type="hidden" name="showMessageTemplates" value="' + input + '" />');
document.send_msg.submit();
}

The problem is that when you click the link, the page simply reloads to a blank page with the only code being the hidden field. This is not what I want, I want the hidden field code to be passed along, and then for the form to submit. So, what should have is that the hidden field code is added, and then the form submits.