Hi Nicola, to add your personal domain based email on your Convertkit account you will need to go to this page HERE18. You will see this section below > Click "add from address" > Enter you new email address> Save it> You will then receive a confirmation email to your new address, then confirmed.
last step is once ti showing confirmed click on "Set to default".
I hope it helps. If you're still having issues or need further assistance you can always reach out to firstname.lastname@example.org.