To improve on nivla's answer, you can right click on almost any input field and select "Add to search bar". This will build the appropriate URL, based on the text field's name attribute and the action of the form which posts the field.

So if I had a form on "example.com" which had an action of "/do-search", and finally, a text input with a name of "query", then the URL it will add is:

http://example.com/do-search?query=%s

Saves you having to inspect the URL yourself to figure out the parameters.

Then you can go to the settings page nivla mentioned and make it the default search engine, so that any non-url's you enter into the omnibox are redirected to the %s in the URL above.

Click on the "Zebra Crossing" (that used to be the wrench/spanner :sigh:) icon > Settings > Search > Manage Search Engines

Scroll to the bottom and add "DDGooG" as Name, duckduckgo.com as the keyword and https://duckduckgo.com/?q=!g%20%s as the URL.

Hover over the URL and select "Make Default".

You are all set :)

I just altered the Google query to only send a q= field. My search URIs look like this: https://www.google.nl/search?q=test.