Onyx does not log into your MetaTrader account. It is the other way round: you install a small program (the EA) inside your MetaTrader, and that program sends us your trades. We never have your password and cannot move your money.
It is a password that identifies your account. You create it in Accounts, paste it into the EA, and that is it. The first time the EA syncs, that key gets bound to the account number and will not accept another.
Every MetaTrader account needs its own. Your plan decides how many you can have active at once. If you hit the limit you can revoke a key to free a slot: that does not delete the history you already uploaded.
Onyx Trading Live