Edit the message to show in the welcome message. *Don´t change {{user}} in case you want the message to say the username.
!set edit welcomeMessage [NewMessage]
[ ] is an argument. You can check all arguments here 👉 #arguments.
< > is a reference. You can tag someone within the same the guild (<@GuildMember>) or the way that a user is logged in a gaming platform (<Username/ID>).
<@GuildMember> can be changed for <username/ID> in any case but not the opposite.