Sign in with your email address and the same password you use for its mailbox (e.g. Ionos webmail). There is no separate Emailer account to create.
Your password is checked with your mail server and never saved. Accounts you add are remembered, locked with this password, so one sign-in opens them all.
Nothing here.
Choose a message to read it.
Add another mailbox to read and send from here. Its password is checked with the mail server, then saved encrypted, locked with your sign-in password, so it opens automatically next time you sign in.