Thanks for the clarification, @StefanoF. The DNS record would be added here.
The title would go in the name field and be only “_domainkey” as the root/apex domain name will be added to that automatically. I’ve included a screenshot which show which fields to use:
The value they give you for the TXT record will replace the “something here”.
If this still doesn’t work or there are other questions, please let us know.