Selecting Input for selected MIDI track in Pro Tools

This script should do it.
Just change the midiInputPath for your desired MIDI Input:

const midiInputPath = ["Predefined", "SoundFlow Custom Midi Output 1", "channel-1"];

/**
 * @param {String[]} midiInputMenuPath
 */
function setMidiTrackInput(midiInputMenuPath) {
    const tracks = sf.app.proTools.tracks.invalidate().allItems;

    const firstSelectedTrack = tracks.filter(track => track.isSelected)[0];

    //Check if selected track is a MIDI track
    if (firstSelectedTrack.type !== "Midi") {
        sf.interaction.notify({ title: "Selected track is not a MIDI track" });
        throw 0
    };

    //Ensure track I/O is visible
    sf.ui.proTools.menuClick({
        menuPath: ["View", "Edit Window Views", "I/O"],
        targetValue: "Enable"
    })

    //Ensure track is visible in Edit window
    sf.ui.proTools.trackGetByName({ name: firstSelectedTrack.name }).track.trackScrollToView()

    // Define MIDI input popup menu button
    const midiIo = sf.ui.proTools.selectedTrack.groups.whoseTitle.is("MIDI IO").first;
    const midiInputBtn = midiIo.buttons.whoseTitle.startsWith("MIDI Input selector").first;

    // Select MIDI input
    midiInputBtn.popupMenuSelect({
        menuPath: midiInputPath
    })
}

setMidiTrackInput(midiInputPath)