How to receive a delegation from a subdomain ?

I use tinydns.

We have a customer. They have a domain named test.com which configured at 
their dns server. They created a subdomain named tr.test.com.
They delegated this subdomain to our dnsservers ( dns.us.com and 
dns2.us.com )
They have 2 dns servers called  ( ns1.customer.net ,  ns2.customer.net )
I want to add an MX record to this subdomain at my dns servers

How should I configure my data in tinydns in order to receive the delegation 
?

Thanks