This makes it so that as long as that button is being held, moving the cursors scrolls instead.

set_ergo_scroll.py

#! /usr/bin/env python

import re
import subprocess


RE_LOGITECH_ID = rb"Logitech MX Ergo[\s\-\w]+id=(\d+).*pointer"


def set_ergo_scroll() -> None:
    """Set button scrolling behavior for MX Ergo."""
    print("Setting ergo scroll...")


    inputs = subprocess.run(["xinput", "list"], capture_output=True).stdout
    ergo_ids = re.findall(RE_LOGITECH_ID, inputs)
    for ergo_id in ergo_ids:
        rc = subprocess.run(
            [
                "xinput",
                "set-prop",
                ergo_id,
                "libinput Scroll Method Enabled",
                "0,",
                "0,",
                "1",
            ]
        ).rc
        assert rc == 0

        rc = subprocess.run(
            [
                "xinput",
                "set-prop",
                ergo_id,
                "libinput Button Scrolling Button",
                "8",
            ]
        )
        assert rc == 0

    print("Ergo scroll set.")


if __name__ == "__main__":
    set_ergo_scroll()