Maybe if nothing is found in the auto-complete field you could have a
default item in italics that says something like "My bike isn't
listed?".
Then when the user clicks that item you show/hide or dynamically
create 2 new textboxes.
One textbox for the manufacturer and the other for the model.
Not sure whether my approach is what you want but there's my 5
cents :)
PS. I tried using the sketch link to comment, but the validation kept
telling me that i have to enter my email address even though I did.
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https://skitch.com/darrenbockman/r7118/template-gear-tracker-v3.gtemp...