The user account you intend to run your bot as, would need to be a moderator in your channel, as only moderators and broadcasters can send announcements.
Then just start your message in the PRIVMSG with an .announce or /announce (I prefer the dot version). This works the same as if a moderator or caster would send an announcement when using native twitch chat.
Chat or bot you both write .announce the message you wish to announce