Hi sudo,
Welcome to deSEC, and thank you for your question!
If I understand you correctly, you already have a dynDNS domain such as example1.dedyn.io, and you would like to add another dynDNS domain such as example2.dedyn.io in the same account.
You can do so using our REST API. Check out our section on Creating a Domain:
curl -X POST https://desec.io/api/v1/domains/ \
--header "Authorization: Token {token}" \
--header "Content-Type: application/json" \
--data '{"name": "example2.dedyn.io"}'
In this command, you need to replace the {token} with the dynDNS “password” you have been using for your existing domain. (The dynDNS “password” is actually an API token.)
If this was not what you meant, please let me know!
Stay secure,
Peter