Why am I not allowed slashes in the proxy setting? HTC/Android 4.4.2

I am trying to set up VPN at work, which requires me to define a proxy server. I can long-tap on the staff WiFi name and enable advanced options, which allows me to define the proxy and port – except that the Save button seems to grey-out or not depending on what I type. It seems to be validating the proxy address as I type.

My problem is that our work proxy server address contains slashes, which the validation doesn't accept.  So, if I enter proxy.pac then I can save it, however, I need to enter http://xxx.com/proxy.pac

Having looked around the forums, I think this must be a fairly rare problem - I see only a few references to this field not accepting a slash (in relation to adBlock). Although this does give me some assurance that its not just me, it doesn't really seem to offer a solution - those forums seem to find either a trailing space at the end of the proxy name, or that entering the form's fields in a reverse order then enables the save button.

So my questions are:


Is this a known problem? Can it be repeated elsewhere? Am I doing something stupid?
Can a 3rd party app offer a solution? I've tried some, eg "Proxy Settings" but it too doesn't allow the slash, also seeming to attempt to validate the hostname setting and refusing to accept what I need to enter.  
Would rooting the phone help me? I see a lot of the available apps for setting proxies require that the phone has been rooted - I am reluctant to do this unless I know it will solve my problem.
I am aware there is a new release of Andriod around the corner... perhaps somebody with hands-on experience can see whether this problem still exists (simply try and enter a string with a slash in it for proxy settings, if you can press enter then I'll be first in the queue to upgrade when its available for me)


Any help/comments much appreciated!
Your http://xxx.com/proxy.pac is not a proxy server, it is the address to a proxy automatic configuration (PAC) file. A PAC file contains a Javascript function that determines which proxy address to use and/or whether you're allowed to go direct when connecting to a particular URL.

ProxyDroid (requires root) claims to support proxy configuration using PAC file (I had never personally used it though).