The ability to define custom searches is fantastic, but currently it's way too difficult to locate in Settings. Currently the user has to go to Settings > Privacy and Services > Address Bar > Manage Search Engines. This is not intuitive, and way too deep in the settings menu.
In Chrome, right-clicking on the address bar will show "Edit search engines" at the bottom of the context menu. Please copy that behavior to Edge, because it makes the feature more discoverable.
And second, in Chrome Settings, "Search Engine" is a top level section in the Settings page. Please consider adding a top level Search Engine settings menu option in Edge as well.
@NicolSD You can already manually add as many as you like. Do a search for let's say the word testsearch on any website, and look at the resulting address in the address bar. In the address bar, replace the word testsearch with %s and copy the edited address. That's your new custom search. You can go to edge://settings/searchEngines , press add, add a name and a custom "keyword" that calls it, and paste the edited address in the URL box. Works for many websites.