Glue Records are necessary to use nameservers for a domain that ends with the same domain name.
For example, ns1.example.com at the domain example.com.
In that case, additional to the name of the Nameserver, the IP address of the Nameserver must be transmitted to the registry.
1. Register the nameservers / glue records
Go to your domain -> "Private Nameservers"
Here you have to register a nameserver in the "Register a Nameserver Name" section so that it is sent to the registry.
To do this, enter the desired subdomain in the 1st nameserver field. Usually, this is ns1, ns2, ns3 and so on, and in the 2nd IP address field, enter the IP address of the nameserver.
If a nameserver has several IPs, you can do this step several times. If you repeat the step, the existing entry is not overwritten, but the new IP is added to the already existing list.
Due to restrictions of various domain registries, it is unfortunately not possible to view a list of the current entries.
If in doubt, you can delete all entries of a subdomain in the "Delete a Nameserver" section.
2. Changing the nameservers
Once you have registered all the nameservers/glue entries, you only need to change the nameservers to your custom nameservers.
Simply go to the Nameservers menu, select custom nameservers and enter the nameservers you have specified previously.
You can set a maximum of 5 nameservers, but you need at least 2.
(Some domain extensions may have different requirements)