Some build-in search engine of Opera have auto-complete feature. How can I add these capability to my custom search engines? `
Maybe the Opera extension AutoComplete is what you are looking for.
This extension provides Autocomplete for the text input box. Data which was input previously can be shared with any sites. And presents words suggested by search engines. = Features * Complete from data you input before * Complete from suggests by search engines
AutoComplete works together with auto suggested words from search engines. There are a couple of search engines out there which support auto suggest beside google, wikipedia and bing. But opera hasn't added them to their default search.ini file so far. Fortunately you can add them on your own.
You will find your search.ini file at one of these paths (after you have added a custom search engine)
The highlighted part is the important part which can be added manually. The Suggest URL must be known. I found some Suggest URLs simply by googling around.
tested list of Suggest URLs for Opera
You find a good explanation how the search.ini works on this blog.