Try javascript: URL

I managed to get what I want with this (pretty nasty indeed) workaround: a javascript: URL that creates and immediately submits the form. This means, when you define your search engine, instead of a URL like this:

http://www.example.com/search?term=%s

you would use this URL/code:

javascript:document.write('<form name="f" action="http://www.example.com/search" method="POST"><input type="hidden" name="term" value="%s"></form><script>f.submit();</script>');

Since it'll most likely be a pretty long string it's better to prepare it in a text editor and then copy-paste it into the URL bar.

Example: This is the search URL for an Italian-English dictionary:

javascript:document.write('<form name="f" action="http://dizionari.repubblica.it/cgi-bin/inglese/find" method="POST"><input type="hidden" name="lemma" value="%s"><input type="hidden" name="sez" value="ita"></form><script>f.submit();</script>');

And here the same thing again, but with the single line manually reformatted into multiple lines to make it easier to read and understand:

document.write(
    '
        <form
            name="f"
            action="http://dizionari.repubblica.it/cgi-bin/inglese/find"
            method="POST"
        >
            <input
                type="hidden" 
                name="lemma" 
                value="%s"
            >
            <input
                type="hidden"
                name="sez"
                value="ita"
            >
        </form>
        <script>
            f.submit();
        </script>
    '
);

P.S. Unfortunately, this method doesn't work when performing search from empty "New Tab" page. This can be fixed by installing "Ultimate New tab" extension.

This functionality doesn't exist yet in Opera Blink (currently on version 19). Remember, Opera went through a major change from Presto (12.x) to Blink (15+), and they're trying to gradually get all that functionality back bit by bit in each new release.

Additionally, there is always the danger that such functionality can be hi-jacked by malware - which is why this functionality was removed in a security fix from Opera 12.14 to 12.15 and wasn't added back in in the latest Presto version, 12.16.

From the Opera 12.15 release notes:

Fixed an issue where the search bar's default engine could be overridden by third-party apps.

So please don't assume it's an "arbitrary restriction" just because you don't like it. ;-) Also, not too long ago, Opera was the only browser allowing you to even have custom search engines. If you're happy to live with that little security hole due to confidence in your firewall/virus scanner, use 12.14 (you can use that alongside Opera Blink and even right-click on a webpage and "open with" Blink or any other browser); otherwise, you'll have to wait until this gets added back to Opera Blink, or until your favourite search engine gets added to the list of "standard" ones. You can always post to the Desktop wish-list forum.