Hello and welcome!
Yes, that’s your service name, the part that appears after the “@” sign in your user ID. prime.kontalk.net is the service name used in our test instance, but you have to change it to a domain you own.
Either will do, but prefer 5222 since it’s the default XMPP port. 5999 was used in the past for compatibility reasons with an old protocol… long story.
You can’t use an account created by a Kontalk production server in your own server. You need to register to your server. For test purposes, you can use the dummy registration provider, which simply accepts 123456 as verification code. It’s the default provider you’ll see in the default config/init.properties.in file:
sess-man/plugins-conf/kontalk\:jabber\:iq\:register/providers[s]=dummy=org.kontalk.xmppserver.registration.DummyProvider
sess-man/plugins-conf/kontalk\:jabber\:iq\:register/dummy-sender=123456
You can see the 123456 configured there. Come back here if you want to use real SMS messages (some hints also here).
Also a quick note on certificate validation: I assume you didn’t create any certificate or GPG key for your server; the system will create both for you automatically, but the server certificate is of course self-signed. That’s why you had to bypass validation within the app (“Disable certificate validation” checkbox). If you plan to open the server to the world (or even to a few people, whatever), you’ll have to generate a valid server certificate and maintain a GPG key for your server (as in keep it safely in storage and backed up). Special care should be taken for the GPG key since that’s the key that digitally certifies user logins.
I hope I answered all your questions.