Omnikey is a very useful Safari extension that allows searching user-selected webpages by entering user-selectable keys preceding the search term. For example, entering "w Los Angeles" in the Safari address bar on my computer and pressing return opens a new Safari window in Wikipedia that's searching for "Los Angeles." This is both easier and faster than navigating to the page you wish to search, entering the search term, then hitting return.
Here's a screenshot of the searches I've added to Safari on my Macs:
Unfortunately, Omnikey doesn't work with Safari 13.
Omnikey is very useful. Are there any other Safari extensions that provide the same functionality?
Best Answer
As OP informed, restarting after reverting using backup restore made Safari accept the Omnikey as an extension. They are on Safari 12.1.2.
The post below may concern those looking for more ways to achieve the same.
Omnikey can be used in the browser by fooling the browser (Safari 12.1.1) into believing that you're developing the extension(that's what I can make of it).
Load it using Show extension builder in Develop menu in the form of an folder which you can get by expanding the .xar version of the extension file which you can get from the site. Note that to get .xar from extension file, simply use rename feature.
Then just hit run. For details about updating previous preferences, or understanding it in more clarity, see original post by georgegarside at
https://georgegarside.com/blog/macos/install-any-safari-extension-macos-mojave/
But for alternatives:
Safari has inbuilt Quick Website Search, you just have to figure out the term that triggers it. Find out more in Safari Preferences → Search tab → Quick Website Search. Results are like this.
Use DuckDuckGo's !bangs .
source:https://duckduckgo.com/bang
https://computers.tutsplus.com/tutorials/how-to-set-up-custom-searches-in-safari--cms-26139 explains in detail those alternates that I figured out.