I'm afraid the short answer is that you can't in HTML - you either have a dropdown OR an input field - you can't have a combo like in VB.
I guess you could write some JavaScript to create an input field over the top of the dropdown if someone has pressed a key and then update the dropdown with that value when they focus away...
Good luck!
Chris.
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Elen sila lumenn' omentielvo!
(all hail JRR Tolkien...)