DNS Tools: How to change the name servers for a subdomain
Step 1:

Log in to your KAS (Technical Administration) panel and click on Tools -> DNS Settings. Click the Edit icon right next to the domain and then click on Add a new DNS record.
Step 2:

Then click on Save and the entry will be made.
Step 3:

In most cases you need to enter at least two NS records. You can now enter the second NS record the same way like you have entered the first one.
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