Steps to create custom nameservers in Plesk.
1) Login to your Parallels Plesk Panel.
2) Enter the domain name in search bar on the left panel.
3) Click on DNS settings icon.
4) Click on Add record.
5) Create “A” records to point your nameservers to server IP Address.
NS1. IN A Server IP Address
6) Then again click on Add Record and create NS as record type, point domain to Nameservers.
7) Please ensure that you have also created child nameservers in your register domain control panel.