Go to Settings/Search Engines/Manage Search Engines. Add a new Search Engine. This should open the “Add Search Engine” window. Fill in as follows:
Name - Presearch
Keyword - :pr
URL with %s in place of query - https://engine.presearch.org/search?q=%s
Save then click the 3 dots to the right and make this your default search engine.
This did it for me.
Also make sure you delete all old “Presearch” entries. For some reason I had to make another search engine (I used Qwant) the default before I could see some of the old (failed) entries.