No one other than the domain owner can modify dns-record.
If your customers have own dns-hosting (independent of primary - above-level domain) they can create new SRV like this:
Host: _ts3._udp
Value: ts.sub.domain.com
Priority: 10
Weight: 10
Port: 9987
TTL: 86400
Thx. I'm already use it.
I thought that CloudFlare has alternative way to working with TSDNS because is strongly litter in your domain zone.
Look at my dns-records xD