Generate an oAuth token as documented.
Then use that oAuth token as documented to make an announcement.
You already stated how to do it here.
You just need to feed in the bots token.
Which you state you already did here as you have a bot token.
I’m not sure what your issue is as everything you need you already described.
You state your are connecting accounts which implies to me you have done oAuth flows to obtain a casters token and a bot token to use. So you should have everything you need.