You need to talk to your domain registrar on setting default nameservers for new domains. Nameservers are entered into the domain dataset. That is different from editing the content of nameserver records. In Plesk you can edit content of nameserver records, and these contents wills be used if your Plesk server is the entered as the nameserver at your registrar's domain nameserver records of the domain. But what your registrar sets up as nameservers when you book a new domain there is up to your registrar, not Plesk.