When the proxy settings are obtained from the network, it is hard to find these values.  Superuser shows a way to get them from the network.  The easiest way is to just surf to the url http://wpad. If this does not work, first try to find the Primary DNS Suffix in the output of ipconfig /all. Try to reach wpda."priamry dns suffix" and if that works, surf to http://wpad."suffix"/wpad.dat.

— see Superuser