Custom nameservers allow you to use your domain name in the nameserver name. For example, if your domain is test.co.uk, you could use st1.test.co.uk and st2.test.co.uk
To do this, login to your Barruko account and click “My domains” in the domain dropdown
Select the drop down to the right of your domain and click “manage nameservers”
Enter your desired nameserver and click “change nameservers”