In Chrome, visit chrome://settings/searchEngines
...You will see a list of all the tab-activated search engines, which Chrome has collected.
I will assume the search engine you want is NOT listed. If your engine IS listed [a problem I have NOT encountered], it may help to delete it, and re-add it using the following instructions:
Click [Add]
Search Engine: <Any friendly name you like>
Keyword : <What you type in the URL bar to activate the search feature>
URL : <The search URL with "%s" where your search words must appear>
Example ...I recently enabled the Youtube dark theme, which broke the Youtube "tab search" in Chrome:
Search Engine: Youtube
Keyword : youtube
URL : https://www.youtube.com/results?search_query=%s
Click [Add] (again)
If you do not know what the search URL should look like: Go to the website in question, perform a search, and look at what ends up in the URL bar ...Your text will be "URL Encoded", EG. spaces will be replaced by +'s (etc) - do not worry about that, the URL encoding will be done by Chrome's search engine for you :)
Example.
Go to Amazon and perform a search for "eneloop AA" which (for me) generates this URL:
https://www.amazon.com/s?k=eneloop+AA&ref=nb_sb_noss_1
...You might notice that if you remove the "&ref..." from the end, the search still works
https://www.amazon.com/s?k=eneloop+AA
...So our Search engine URL could be:
https://www.amazon.com/s?k=%s