It is possible to put an external url link inside USS login page?
Customizing any USS page is a very manual process, and not at all Supported.
If USS is integrated with Service Desk perhaps an idea you can use is to create an Announcement that will display to provide a URL to customers to click (or even multiple helpful URLs)
Do you have any additional questions on this topic?
If not, please mark a provided response as correct so that this thread can be closed.