I’m not sure what your after but are after code for drop down menu for a form, then when an option is selected it will open a new window opening a website (or what ever, e.g. - a banner)?
This sounds more like a JavaScript thing. Try looking into onChange/onFocus events, they should be able to achieve what you’re after.
For example (not tested, may contain mistakes!):
You JS function should then look something like this:
function changeCode(siteID) {
switch(siteID) {
case 1:
document.getElementById('yourOutputElement').value = '<a href.....';
break;
case 2:
document.getElementById('yourOutputElement').value = '<a href.....';
break;
default:
// This will get executed if none of the above is selected
document.getElementById('yourOutputElement').value = '<a href.....';
}
}