Best way to do it is to set the keyword to yt and type yt, a space, your search term(s) and press enter. You can set the keyword to youtube.com instead if you want to kind of simulate how Chrome does things. However, always use a space after the keyword in Opera. Never use a tab as the behavior is buggy. Also, at the URL opera://settings/searchEngines, keep it set to "space or tab" and just pretend it only says "space". And, pretend the separate tab-only option doesn't exist.
