External integration | include wp-blog-header

Basically I’m trying to include wp-blog-header.php in WHMCS header template file.

{php}
require(‘/path/to/wp-blog-header.php’);
{\php}

After adding that code the https://www.url.com/billing/ redirects to https://www.url.com/billing/wp-admin/install.php

Anyone knows what I’m missing?

Thanks