I know there's already lots of topics about this and i think i read them all but i still can't get it to work. 

I need to update the choosen value from my select to a hidden input field. 

When i try this code for example it doesn't work. When i check the source code the value is still empty. 

What am i doing wrong? 

 <form>
    <select id="dropdown" name="dropdown" onchange="changeHiddenInput(this)">
        <option value="j.hotmail.com">Jens</option>
        <option value="a.hotmail.com">Adam</option>
        <option value="d.homtail.com">Dan</option>
    </select>
    <input type="hidden" name="hiddenInput" id="hiddenInput" value="" />
 </form> 
    function changeHiddenInput (objDropDown)
    {
        var objHidden = document.getElementById("hiddenInput");
        objHidden.value = objDropDown.value;
    }   

Lot's of thanks in advance!