Set up your Nextcloud Notes settings before trying to export with that method for the first time (the plugin will not allow the export method to be selected before you enter your data, so you should land at this point naturally).

You'll need three things. First, the Nextcloud server URL and your username (usually an email address) – those, you should already have.

Next, go to your Nextcloud server, open your settings, then head to Security → Devices & Sessions (at the bottom of the page). Create a new app named KOReader (the name doesn't matter though). Once the app has been created, you'll be shown an app password. Enter the app password in KOReader, and you should be good to go.

You can edit the config manually in your settings.reader.lua, then add / edit the exporter section. Make sure you have markdown enabled as well. sample config :

["exporter"] = {
        ["markdown"] = {
            ["enabled"] = true,
            ["formatting_options"] = {
                ["invert"] = "bold",
                ["lighten"] = "italic",
                ["strikeout"] = "strikethrough",
                ["underscore"] = "underline_markdownit",
            },
            ["highlight_formatting"] = true,
        },
        ["nextcloud_notes"] = {
            ["enabled"] = true,
            ["host"] = "https://nextcloud-instance.com/",
            ["password"] = "pw from Security → Devices & Sessions",
            ["username"] = "username",
        },
    },