Click on Nameserver on the left side of the customer area and afterwards on the eye icon to modify the DNS entries of the respective domain. If SRV entries already exists, you can modify these there.
To add a new SRV entry, please follow the subsequent steps:
Explanation on an example for:
_sip._tcp.example.com. 86400 IN SRV 0 5 5060 sipserver.example.com.
Click on the button Add DNS entry.
Enter _sip._tcp into the field Name.
Choose SRV for Type.
Enter 0 into the field Prio,
Enter 5 into Weight,
Enter 5060 into Port,
Enter sipserver.example.com into Server and
Enter 86400 into TTL.
Save to complete the changes.