We do not offer vanity/white-labeled nameserver service at this time. This feature was disabled as we had reports of unexpected DNS behavior while using vanity/white-labeled nameservers. 

You can still create your own nameservers for domains that are registered in your Openprovider account if you have DNS servers. 

You can do nameserver object registration (for domains registered in your Openprovider account) at registry from  Domain management > Nameserver management.

Note: Please be aware that creating nameservers using Nameserver management way will not make them accessible in the DNS and it is only intended to provide functionality for registering nameserver objects at a given registry. You can read more about it in this article.