If you invited Echo for the first time in the server, the default prefix is $
If it does not work for you, please mention the bot where it can read and send messages.
To change the prefix for your server, just send [prefix]set serverprefix <new prefix>
Here, [prefix] is the prefix for the server and <new prefix> is your desired prefix for the server.
If you have any problems understanding the command, feel free to join the Support Server.
See all the commands from the link below: