The iRig BlueBoard from IK Multimedia is a wireless MIDI foot pedal that can be configured to work with a wide variety of apps including OnSong. Because this is a MIDI device, you will need to purchase the MIDI add-on. To do this, open the Utilities menu (gear icon) → Add-Ons and purchase the MIDI add-on.
Once completed, you would connect your iRig BlueBoard by opening the BlueBoard app installed on your device. The iRig BlueBoard sends program changes on all MIDI channels by default. Tap on the "ALL CH" button at the bottom of the BlueBoard app and change to a single channel. This will avoid some confusion when customizing the foot pedal in OnSong.
Next, open OnSong and choose Utilities → Editors → MIDI. You will see a window with no MIDI events. Press a foot pedal on the BlueBoard to have it appear in this list. You can then tap on that event and assign it to an action. We recommend assigning pedals such as "Scroll Down" and "Scroll Up" as those are the most flexible options. When done, tap "Close".
You can now use the iRig BlueBoard to scroll your music. Of course you can map other pedals to a wide variety of actions as needed. You can also attach up to two expression pedals to the iRig BlueBoard and use them with OnSong as well. You can configure foot pedals further in the foot pedal configuration tool by tapping the gear icon in the lower left of the screen.