Hi Christopher,
Sorry for the inconvenience.
Actually, our HostWHMCS template is an HTML template and we just provided support by customizing WHMCS header and footer and color based on our template for FREE of cost with our template.
However, If you want this type of modification/customization then you have to know the WHMCS core (advanced level).
Please check out these reference links below to solve the issue.
https://squidix.com/documentation/lavenderhost/integrating-a-domain-checking-form-with-whmcs/
https://forum.whmcs.com/showthread.php?15411-Add-Domain-Checker-and-Transfer-to-Home-Page
http://forum.whmcs.com/showthread.php?23587-Free-Domain-Tab-Menu-Whois-Transfer-Search
https://forums.whmcs.com/showthread.php?25701-Domain-checker-question
http://docs.whmcs.com/Linking_to_WHMCS
https://www.youtube.com/user/whmcs/videos
If you are not able to customize it by yourself then you need to hire someone to customize it for you.
Note: Now we are on a vacation of Eid-Ul-Adha till 18 September. So, it can be late to reply.
Best Regards,
ThemeLooks Support Team