I don't have any experience with Windows Surface or the Surface Pen but you might want to check it's settings. I'm guessing the OS is Windows 10 so it should be under Settings>Devices>Pen & Windows Ink then check what you can toggle under pen settings. AFAIK Windows will want you to just toggle settings to it's programs but maybe it has a means of changing shortcut settings for other programs.
As far as Medibang goes you can always check shortcut setting under File>Shortcut Settings but I think it'll only let you select keys on the keyboard.
Sorry if I seems uncertain but I hope you can figure it out 