PDF invoice issue with integration


There are invoices in WHMCS. The integration shows the invoices integrated in the design or our site - nice. But there is a link to get the invoice in PDF:
<p align="center" style="font-size: 14px;"><a href="dl.php?type=i&id={$invoiceid}" target="_blank">Print PDF</a></p>
It is shown OK and has such url:
But when you press the link - you are redirected to:
and the login/pass is asked (despite the fact that you was logged in already at our-nice-site.ru). This is not nice - we would like our clients don't need to log in again.

P.S.- if you are logged in also at our-whmcs.ru (like an admin) - everything is OK - you get the invoice. But if you log out at our-whmcs.ru (still staying logged in at our-nice-site.ru) - you will have to log in again...

Could you fix this pls?

P.S.- I am not authorised to give access, sorry.