<option value="5">Option 5</option>

<input type="hidden" value="5">

function mySelect(x)
{
 document.getElementById("equipment_" + myCounter).value = x.options[x.selectedIndex].value;
 }

<script language="JavaScript">
function doSelect(x)
{
 document.getElementById("equipment_"+count).value = x.options[x.selectedIndex].text;
 document.getElementById("equipment_"+count+"h").value = x.options[x.selectedIndex].value;
}
</script>

function addField(area,field,limit) {
 if(!document.getElementById) return; //Prevent older browsers from getting any further.
 var field_area = document.getElementById(area);
 var all_inputs = field_area.getElementsByTagName("input"); //Get all the input fields in the given area.
 //Find the count of the last element of the list. It will be in the format '<field><number>'. If the 
 //  field given in the argument is 'friend_' the last id will be 'friend_4'.
 var last_item = all_inputs.length - 1;
 var last = all_inputs[last_item].id;
 var count = Number(last.split("_")[1]) + 1;
 
 //If the maximum number of elements have been reached, exit the function.
 //  If the given limit is lower than 0, infinite number of fields can be created.
 if(count > limit && limit > 0) return;
 
 if(document.createElement) { //W3C Dom method.
  var li = document.createElement("li");
  var input = document.createElement("input");
  input.id = field+count;
  input.name = field+count;
  input.type = "text"; //Type of field - can be any valid input type like text,file,checkbox etc.
  li.appendChild(input);
  field_area.appendChild(li);
 } else { //Older Method
  field_area.innerHTML += "<li><input disabled name='"+(field+count)+"' id='"+(field+count)+"' type='text' readonly />";
  field_area.innerHTML += "<input name='"+(field+count)+"h' id='"+(field+count)+"h' type='text' readonly /></li>";
 }
}