Falkon's search engine manager has a list of shortcuts (google g, wikipedia, w, duckduckgo d)
How do I input these shortcuts correctly in the search bar?

Fiddling with it a bit, looks like you just start your query with the shortcut followed by a space followed by your query into that engine.

e.g. to search wikipedia for Toronto, enter "w Toronto" into the url bar. The highlighted entry should say that it will search for it on Wikipedia.