[jQuery] hidden form field

How do I insert a value to hidden form field "user_name"?

<form name="myForm" id="myForm" method="post">
<input type="hidden" name="foo" value="bar"/>
<input type="hidden" name="user_name" value=""/>
</form>

jQuery('form#myForm input:hidden ???').val('fast_fish');

Thats what I need:
<input type="hidden" name="user_name" value="fast_fish"/>