ok so this is a little fiddly but bare with me...
first, set the game to borderless windowed mode, and set the resolution to that of your secondary monitor - this can be done from the options menu in-game.
then, in your options.cfg file there is a setting for windowedModePositionX. This setting dictates where the window will initially open. Set this to the screen position relative to the top left corner of your primary display source, ie: if the desktop extends to the right of your main monitor and your main monitor is 1080p, set windowedModePositionX to 1920 (maybe 1921) or if it extends to the left, and your secondary display is 1440p, set it to -2560 (maybe -2561)
you may also need to play with windowedModePositionY but that should be fairly straightforward.