An explanation of proxy use in PC Linux OS would be helpful.
When do you use IP:Port vs //server/folder/file 
Are you looking to run a proxy (server) in PCLOS, or access an existing one?
With regards to
IP:Port vs. //server/folder/file, they are very different...
IP:Port is used at a "lower" network level (the "IP" of "TCP/IP"..."TCP" is "Transmission Control Protocol", and "IP" is "Internet Protocol"...they are the "carriers" of higher protocol layers, such as HTTP, FTP, SSH, SMB, NFS, etc)
//server/folder/file is used at the "higher" levels, such as HTTP (web browsing), SMB (Windows file sharing), etc
For the IP:Port scenario, the Port would be specified if a server process (such as a webserver, or file server) is listening on a non-standard port. When a standard port is used, it is assumed to try it (so, for a web server, which usually uses port 80 is accessed, for instance, via
//server/folder/file, the computers assumes you mean
//server:80/folder/file). If you decide to run your webserver on the non-standard port of, for example, 12345, then you need to explicitly tell the computer to access
//server:12345/folder/file, otherwise the request will not go the the correct process running the webserver located on
//server/.
I know this rambles a bit, but maybe it will help explain it some. If you want more detailed explanaations, just ask for clarification of the points you want explained...