Add Options to a Combo Box Using JavaScript

You can use a JavaScript function to modify the options available in a pull-down box on the page.

The following example adds a new blank selection at the top of the starthour field at Level 2:

try{
 document.forms[0].starthour.add(new Option("", "", true, true), document.forms[0].starthour.options[0]) 
}
catch(e){ //in IE, try the below version instead of add()
 document.forms[0].starthour.add(new Option("", "", true, true), 0)
}