Please read these articles for setting up External MIDI Devices with Studio One 3:
Also, make sure the XP50 is set to external MIDI clock.
1. For the simple use of the piano keys of the XP50, you will need to create a "New Keyboard", with the "MIDI Receive From" assigned to the MIDI port the device is connected to. You don't need the MIDI "Send To" set to anything. Set this to Default Instrument Input. With this option set, if you choose you can add Presence to a track and the Roland keys will trigger it.
--- If you want to do more advanced MIDI continue --
2. To use the Sounds of the Roland, you will need to create a New Instrument with the MIDI "Send To" set to your keyboard MIDI port. Check all the MIDI clock options.
3. To use the controls of your Roland, you will need to Create a New Control Surface with the MIDI Receive From assigned to the MIDI port the device is connected to.
Then check out this article on setting up to record using an External Device: Setting up to record with an External MIDI Device Studio One:
We also encourage you to create a thread about this on the Presonus user forum. https://forums.presonus.com/viewforum.php?f=108
This will allow for back and forth discussions about an issue or question and more points of view as to possible different approaches or workflows.