GarageBand doesn't natively support MIDI output to external synths, but for GB on iOS apparently there is an app called RouteMIDI that will let you do that. From the App Store page: "RouteMIDI, in conjunction with a DAW such as GarageBand, will allow you to create tracks that will use any of your external MIDI synthesizers for that track, as alternate to instruments etc. built into the DAW. For each track that you select RouteMIDI as it's Audio Unit, you will be able to browse through whatever external devices are connected. Then, you may optionally direct the MIDI to any of the standard 16 Channels so that Multi-Timbral units are supported." Here's a link to the App Store page [preview]: https://apps.apple.com/us/app/routemidi/id1347099452 I don't have an iOS version of GB so I can't vouch for this app or offer any details on its operation, but it seems to be recommended frequently as a solution for getting MIDI out of GB iOS. Note: The Alternative would be to record sound from the P-125 on an Audio track instead of recording the MIDI data on an Instrument track. You would not have the same access to the MIDI note editor that way, but the audio would be part of the Project [you'll probably have to do that eventually anyway so you can play the Project without having to have the keyboard attached]. PS - Please try to avoid posting multiple versions of the same post