Your domain is automatically appended to any record.
If you want to create a record for _dmarc.mydomain-isthis.co.uk, you should only enter _dmarc as the name.
_dmarc.example.com would have been turned into _dmarc.example.com.mydomain-isthis.co.uk automatically, only your domain isn’t shown in the dashboard, as it it obviously the same for any record.