I am not aware of an option to post one announcement to multiple courses. The work around is to create the announcement in one class. Once created, go to Announcements and click on the announcement itself. This should be the same view you would see after creating it. There is a 3-dot menu on the far right that allows you to Copy To another course. Just copy it to each course. Once it arrives in the destination course, it will have the same settings for dates as the original.
