The mid and post-credits scenes for Justice League are now online.
You can watch the mid-credits scene in the section above and the post-credits scene below, both courtesy of YouTube channel Filmic Box.
The post-credits scene, on the other hand, was filmed by Snyder and is the one that teases and sets up Justice League 2 (or, at least, Snyder’s planned version of the sequel).
In the scene, Lex Luthor recruits Deathstroke to join the Legion of Doom, thus setting up a potential supervillain team-up in the future to rival the newly-formed Justice League.