As of May 2017 it is not possible in the customer page to create DNS glue records at United Domains.
To create dns glue records contact United Domains support by opening a new ticket.
State your domain name and give the information for the dns glue records in the format of:
ns1.test123.com 127.0.0.1
ns2.test123.com 127.0.0.2
Contact Info:
United Domains
New Domains. New Opportunities.
210 Broadway, Suite 201/NGIN
Cambridge, MA 02139
United States of America
Phone: +1 (617) 863-3812
Fax: +1 (781) 538-0476
Email: support AT uniteddomains.com