Goto the URL opera://settings/searchEngines, click the edit icon next to that custom search.
Make sure the URL is set to:
https://www.celebritymoviearchive.com/tour/search.php, check the "POST" box and enter:
searchstring=%sfor the post query string field.
If you right-click in the search field on the site and choose "inspect element", you'll see method="post" for the form and you'll see name="searchstring" for the text field.
The alternative to the above is to not use POST and set the URL field in the search engine's settings to:
https://www.celebritymoviearchive.com/tour/search.php?searchstring=%s