Announcments are broadcast to chat as a USERNOTICE with a tag msg-id of announcement
Here is an example
@badge-info=;badges=broadcaster/1,ambassador/1;color=#033700;display-name=BarryCarlyon;emotes=;flags=;id=d2c97aaa-b921-45ca-b1f5-df6bbcedb289;login=barrycarlyon;mod=0;msg-id=announcement;msg-param-color=PRIMARY;room-id=15185913;subscriber=0;system-msg=;tmi-sent-ts=1665265341857;user-id=15185913;user-type= :tmi.twitch.tv USERNOTICE #barrycarlyon :test