Easiest way ... go to the page with the search field, then long press the search field.. You'll get a menu that includes "Add search engine ..." - remember to add a unique shortcut to it; as I recall it ignores options with no or duplicate shortcuts.
It is a question related to how the parameters are passed to the site. If you don't know, try leaving it as it is first. If that doesn't work, you can always try again and change it.