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 !Go through the video tutorial to get the above discussed procedure in detail. You may read the blog as well.