How to write your own search for your website from URL

I have seen that you type domain name to the url and put a space, then the website automatically waits for you to enter the string to be searched.

http://i.prntscr.com/g0-tLmvhTCCnBwyh7J0zKQ.png

For example: type youtube.com to the URL bar from your browser then put space character then type your search string.

How can I do that for my website?