Freshchat works on WHMCS. It is quite easy and basic. All you need to do is use FTP or open the 'header.tpl' file on the WHMCS directory then navigate to 'templates/six', depends on what template name you're using, by default is 'six', then edit header.tpl, after that paste the first code ( within HEAD part ) on Paste that code within the <HEAD> section.

Then save, after that, open the footer.tpl file, paste the 2nd code ( before /BODY) on the, Paste that code before the </BODY> section.