First, click on “Forwarders” to arrive at this screen:
From here, you can enter an address you’ve already created (email@example.com) and choose a destination to forward those emails to.
Be aware, though: When you answer those emails from your other account (like firstname.lastname@example.org), they will come FROM that account, not from an email associated with your domain name.
If you want to send emails from email@example.com, you need to either log into your webmail, or configure your Mail Client and MX Records accordingly. This is beyond the scope of this guide.