Fires event before setting username, allows listeners to override it.

This is a hook to override the username that will be used when authenticating token users (which are using anonymous login with auto-generated username).

resources/prosody-plugins/mod_auth_token.lua

 function provider.get_sasl_handler(session)
 
 	local function get_username_from_token(self, message)
-        return token_util:process_and_verify_token(session);
+        local res = token_util:process_and_verify_token(session);
+
+        local customUsername
+            = prosody.events.fire_event("pre-jitsi-authentication", session);
+
+        if (customUsername) then
+            self.username = customUsername;
+        else
+            self.username = message;
+        end
+
+        return res;
 	end
 
 	return new_sasl(host, { anonymous = get_username_from_token });
 	-- This calls the handler created in 'provider.get_sasl_handler(session)'
 	local result, err, msg = self.profile.anonymous(self, username, self.realm);
 
-	self.username = username;
-
 	if result == true then
 		return "success";
 	else