If Steam Guard is used, you need the STEAM_AUTH_CODE secret. Follow this url as a guide:
https://github.com/SteamTimeIdler/stidler/wiki/Getting-your-%27shared_secret%27-code-for-use-with-Auto-Restarter-on-Mobile-Authentication#getting-shared-secret-from-ios-windows
You need to either switch to Steam Desktop Authenticator or extract the Shared Key secret from your phone's files.
If Steam Guard is not used, STEAM_LOGIN and STEAM_PASSWORD will be enough.
Use any broswe and login to NexusMods. F12 and go to Application tab. Find the Cookie tab in the Storage section. Copy the value of the nexusmods_session cookie. This is our NEXUSMODS_SESSION_COOKIE secret.