To do this, you need to add NS-records for the subdomain name pointing to the host names of the DNS servers hosting the subdomain - in the parent zone.
In the "DNS records" window, right-click the parent zone in the left list and select "New NS-record":
In the "New NS-record" dialog, enter the subdomain name and the host name of one of the DNS servers hosting the subdomain:
Repeat the previous steps so that you have an NS-record for each DNS server hosting the subdomain:
Note that it is important that matching A-records exist for DNS server host names listed in these NS-records.
For the example above, A-records must exist for "ns1.otherdomain.com" and "ns2.otherdomain.com" - in the "otherdomain.com" zone on whatever DNS server is hosting "otherdomain.com".
If the DNS server host names are themselves sub-names of the domain name being delegated, it is necessary to include a copy of these A-records in the parent zone. These are called "glue records".
Otherwise other DNS servers will have no way to find the subdomain's DNS servers, and therefore no way to resolve records in the subdomain.
In the screenshot below, the highlighted A-records are such "glue records":
