ConnectApi.FeedElement feedElement = ConnectApi.ChatterFeeds.postFeedElement(Network.getNetworkId(), '0F9d0000000TreH', ConnectApi.FeedElementType.FeedItem, 'On vacation this week.');
The second parameter, subjectId is the ID of the parent this feed element is posted to. The value can be the ID of a user, group, or record, or the string me to indicate the context user.