Log in to IONOS and go to the Domains & SSL section.
Click on the Gear SYMBOL for the desired domain under Actions and then on DNS.
Click ADD RECORD and select NS under Type.
Enter the desired subdomain in the Hostname field.
For Dynamic DMARC enter: _dmarc
For Dynamic DKIM enter: _domainkey
In the Point to field, type the host name of the name server.
For Dynamic DMARC enter: ns-dmarc.ondmarc.com
For Dynamic DKIM enter: ns-dkim.ondmarc.com
Optional: Select the desired TTL (Time-To-Live).
Click Save.
Create a free OnDMARC account to test your configuration.