How to use a non default DKIM identifier?
To use another non default DKIM identifier you can create a record in the DNS with the desired identifier, for example "domreg", so the record becomes: domreg._domainkey.[your domain.ext].
When this record is made, please contact support (email@example.com) and state the record you made. Support Team will then change the default for your domain.
Note: This is a new functionality, and not visible in your control panel. However, if the record is made correctly it is possible to validate your domain in the control panel.
For more information about how to create a DKIM DNS record, please go here.