I bought an account on a website and received the email address @elitegamers.club, but I've never had any contact with that type of email. I don't know anything about plugins. Is it possible to change the password?
If you only have access to the Roundcube account you will not have access to add any plugins. If there isn't an option to change the password in the settings then you'll need to contact the person who you bought the account from and ask them how they support changing the password.