oh i wasn't fully pay attention. you are right you can't choose custom search engine as default.

But you can add custom search engine and select it as default.
Go to "Settings (LAlt + P) > Search Engine > Manage search engines > Other search engines > [Add]" . After you added you need to go "Settings (LAlt + P) > Search Engine > Set which search engine is used when searching from the combined search and address bar" and chose your.