You can add the http-proxy line to your Config.prc file. In the case of packp3d (and other .p3d files), the Config.prc file you need to edit is the one you create in the Panda3D/prc directory (e.g. C:/Users//AppData/Local/Panda3D/prc on Windows, or ~/.panda3d/prc on Linux). Create a file in this directory called Config.prc, and add the following line: