It's really not up to me to let you do that; it's up to the server.
From my end I have to believe the URL information the server is giving me via PHP. However, the server is not passing the correct information to PHP, which is the whole root cause of the problem.
Those temporary URLs were conceived back when people where deploying static HTML sites with the occasional CSS2 (not a typo!) stylesheet. We're talking late 90s, very early 00s. As sites became a lot more reliant on PHP and evolved into outright web applications with their own ecosystem of extensions neither Plesk nor cPanel caught on.
The easiest way around this limitation is to buy a very cheap domain and use it instead of a temporary site URL. There are of course many other ways around it (including using an invalid domain name mapped to the server's URL in your computer's hosts file) but they get increasingly convoluted and acquire enough moving parts to become a hindrance.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!