In ZS 3.0 it’s possible to change the protocol part after creating a new SRV resource record.
First create a new SRV RR and pretend it’s going to be _tcp at first. Save it. Now select it, and in the Entry Name field replace _tcp with _tls and Save.
You can also change the entry name if you create a new entry, but any field you change in the spaces below would put the _tcp part back. Just make overtyping _tcp with _tls the last thing you do before hitting Save.