Registrar issue with custom email domain

Hi, my domain registrar gives me error when I try to register this domain.

Name: sig1._domainkey

Type: CNAME

Data: sig1.dkim.fuatsengul.com.tr.at.icloudmailadmin.com.

Error: Must be valid DNS name

I have also tested without the dot at the end got the same result. The issue here is that the subdomain count is too high for them. I have defined other domains with same registrar and those domains didn't have .tr root which I believe this is why I get this error.

Is there any alternative for this record definition because it's impossible for me to pass verification otherwise.