Below mentioned steps will explain the procedure to add domain search form on your WHMCS site.
Step::1 Login to your WHMCS.
Step::2 Hover over the Utilities section and click on Integration code option.
Step::3 Copy the code from the Domain ordering box.
Step::4 Now login to your cPanel and go inside File Manager section.
Step::6 Now go to WHMCS directory and paste the code inside any page of your website where you want this domain search box to be displayed.
That's It !