Select Proxy Servers... from the Preferences menu.
If HTTP is already specified, and checked, then you need to write down the host and the port that it is set to use. The host is the name before any :, the port is the number after the :, or 8080 if none is specified. Later you will use these to tell WebMask about this existing proxy.
Make sure HTTP is checked, and then enter 127.0.0.1:8080 into the HTTP edit box.
Setting up your browser to use WebMask