To insert integration code connect FTP/cPanel/etc and access your WHMCS files. Go to this path /whmcsdir/templates/clientx-child/includes/ and find head.tpl and edit this file. Copy and paste your integration code somewhere between <head></head> tags and insert integration code with smarty tags {literal}{/literal}.
Note. whmcsdir is your WHMCS installation folder. It could be under public_html or under any subfolder.
Example:
<!-- Global site tag (gtag.js) - Google Analytics -->
{literal}
Your Google Analytic script goes here
{/literal}
<!-- End Google Analytics -->
Note. Do not forget to use smarty tags {literal}{/literal} otherwise your website will crash and show empty page.
