Port number on https web site

i can reach an address from my work site which goes through a proxy server.
https:siterequired.org.uk
but as soon as i try
https:siterequired.org.uk:7788
I can’t access it.

However from my home broadband i can access both the address’ above using the same browser I.E 7.

This one’s got me stumped as the port is obviously open. Can anyone assist me on this one.