How can I find out the URL for a website's search engine?

I'm learning about some of the features in the Edge browser, and one is that you can enter the search engine for a website, such as Amazon or Audible, and then assign it an abbreviation and search it from the address bar.

For example, I want to find out the url for the simple keyword search on Bookshare.org.

Once I have the url, I can enter it in Edge's search engine settings, assign it an abbreviation such as bk, and then, when I want to use the simple keyword search on Bookshare, I can just press control+l to open the address bar in Edge and type something like this:
bk beowulf

and Edge will return results from Bookshare that match beowulf.

The stumper is that that simple keyword search appears on every page of the Bookshare website and, when I use control+u to view the code on a Bookshare webpage, it's all scripts and divs and nothing that looks like a url for the keyword search.

The person giving the tutorial made this sound as if it would be obvious, but didn't provide any tips for how to find out what that url might be.