First of, login to your webmail and go to your mail settings (bottom left icon)
When in Settings, go to "Identities" tab, click "Add an Identity" and fillin the form. Once done, hit "Add" button. (Every disroot user by default has an firstname.lastname@example.org alias he can use)
You can manage default identity, by simply dragging the identity to the top of the list.
To send email with your new alias, just click on the "From" field and select alias you want to use from the dropdown menu, when composing your mail.