Set dropdown value in a Javascript bookmark, my code works with textfields only

At work we had a handy coworker who made a bookmark that auto-filled our ID in textfields on a webpage using javascript that worked wonders. However my company changed the textfields to a dropdown and it no longer works. I've been digging around for a while and I can't seem to find a solution that works. I have very little experience with javascript though. This is what the code was for the textfields:

javascript:{document.getElementById('ID').value=%12345%22;document.getElementById('vendor').value=%2212345%22;void(0)}

I'm guessing the problematic part is the ".value=%" part? Do I need to put something else for a dropdown list?