What is the server in a Client application of Spark 2.6.3

I have downloaded the spark 2.6.3 from the website. It is a messenger application.

There is a user registration form. Where I wrote username & password. But I dont know what is to be filled in the option “server”

First of all, why do you want to use Spark? If you want to chat with some known messengers like Yahoo, ICQ, AOL, Live, Skype, etc., then it will be easier to use their official clients.

Anyway, Spark is a client program and it can’t work by itslef, it needs a jabber/xmpp server to connect to. So, you have to either setup your own server (like Openfire from this website, or any other jabber/xmpp server) or use one public jabber/xmpp server, like jabber.org or igniterealtime.org or else. You should be able to create an account on these servers. Then you should add some contacts. They have to be jabber contacts, if you have such. You can also add your other networks accounts to Spark (like Yahoo, ICQ, etc.), but your jabber server has to have Kraken plugin installed and confifured to provide such options.