Unfortunately, you can't. If you use Email Forwarding, people can send emails to firstname.lastname@example.org, and they will be forwarded to another email address of your choice where you will be able to check and read them. You won’t be able to send emails from email@example.com since it is a virtual email box.
In order to both send and receive emails using firstname.lastname@example.org, you will need to set up a full-fledged email account. You may review our available email plans to choose which one fits your needs; if you already host a domain on our web hosting plan or with another company, email might be included into the hosting package in question.