Log in to IONOS.
For the desired domain, click on the gear symbol under Actions and select DNS.
Click Add Record and select TXT under Type.
In the Host name field, specify the desired host, For example, enter @ for an SPF record, or enter the value _dmarc for a DMARC record.
In the Value field, paste the value you are setting as the destination of the Host (i.e. the value of the SPF or DMARC record to be used).
Optional: Select the desired TTL (Time to Live). By default, your settings are immediately active.
Click Save. The TXT record has now been added.
Create a free OnDMARC account to test your configuration.