Necro-ing this because a year and a half later and this is still not a thing.
We do have the option to use tab for that in the configurations... Only it doesn't work.
#foralltosee: In the "Manage search engines" menu, arrow pointing to the "Tab" option for the configuration "Keyboard shortcut: In the address bar, enter the keyword for the website you want to search. Then, use a keyboard shortcut to continue." Arrow leads to the subtitle: "Doesn't work: Tab just cycles through similar browsing history and default search engine suggestions. Yes, I *am* trying both the default search engines' keywords and my custom configured "Other search engines", which are basically DuckDuckGo as "duck" and YouTube as "yout" (got used)."
Opera advertises itself as the "Fastest, safest and smartest" browser... Yet, Chrome is currently ahead of it in the "fast" and "smart" aspects, in this particular feature. And considering this pops back up every now and then since 2016, I honestly think it would be very nice to see it getting implemented.