forwarder channels are typically installed in WAD form. There are some exceptions that have an actual forwarder channel installer (ie. WiiMC and other apps by Tantric).
Make sure u have some brick protection in place before installing random WADs u find on the net. Priiloader should be enough to recover from a bannerbrick, but make sure u have a nand backup and bootmii installs as well.
To autoboot a file from priiloader you don't need a forwarder channel installed, but u can just install a forwarder dol instead. You can use ModMii classic to build forwarder dols that launch an app off an sd card or hard drive (you can customize the paths), or a URL via the internet channel (customize the URL), or launch a channel installed to your system menu (customize the channel to launch, this could also be used to launch forwarder channels but it's kind of redundant). Anyways, build the forwarder dol u want via ModMii's advanced downloads page, then save it to the root of your sd card, start priiloader, install the dol, change your autoboot settings to the installed file and your done.