It depends on how HD HomeRun decides what the default application is. If it picks the application from the default xdg list, you should be able to change it using settings, or the xdg-mime tool. If it requires you to configure it in its configuration file, you'll have to supply the information there.
I'm not entirely sure if Videos supports streaming without the installation of additional plug-ins, so that may be why it doesn't work. With VLC or MPV, the command is simply vlc <URL> or mpv <URL>.