The company I am entering the cname for (Ringba - and established saas) has told me it needs to save the full domain name and instructed me to contact Cloudflare.
As @i40west said, you should enter only _0fbdfdf2a699314cfcbc2da2219cf172.portal.mydomain.com or _0fbdfdf2a699314cfcbc2da2219cf172.portal, not CnameValidationEntries":{".
That is the correct behaviour. As every record you have ends in your domain, Cloudflare doesn’t show the domain at the end of records to make the names shorter in the dashboard.