It kind of tells you exactly. So like in the screenshot you provided
Search Engine = Name of the search engine you want to add
Shortcut = The shortcut you would like to use to search it. For example, maybe you want to type in :bs and it would default to Brave Search. Then maybe ;g for Google. It’s whatever you want.
URL with %s in place of query = Exactly what it says. So do a search on your website and then copy/paste the URL. You’ll notice in the URL that has a query, which is your search term. Replace that with %s instead.
For example, I went to DuckDuckGo and typed in Somewhere. It then navigated to https://duckduckgo.com/?q=Somewhere&ia=web. So I would remove everything after the =, which is my query. This means URL I’d be putting into that part of Add search engine would be https://duckduckgo.com/?q=%s