post = new FeedItem(
    Title = 'Test - Post Title',
    Body = 'Feed body',
    ParentId = '0F96C000000GzeoSAC',   <---- Community Group ID needs
                                             to be dynamically filled
    NetworkScope = '0DB1U000000Cc9ZWAS' <---- Community ID needs
                                             to be dynamically filled
);
feedItems.add(post);