Step 1:

            Create account  or sing in to Google Custom Search at https://goolge.com/cse

Step 2:

            Select  New search engine 

Step 3:

            Create your search engine for your website or webpage.

Step 4:

            Select  and copy search engine code from here. 

Step 5:

            You already created search engine code, your can get your Google search engine unique id from your control panel. Select your search engine and copy Google search engine id. 

Step 6: Zoho Sites's Search Box is available at Manage-->Settings-->Search Box and paste your code and choose the show option.