As for how we might see Darkseid in Justice League, it was reported last year that there will be a flashback set tens of thousands of years ago depicting an epic battle between Darkseid’s forces and the humans, Amazonians and Atlanteans on Earth, one which ends with the Apokoliptan warlord leaving behind three Mother Boxes.
One of those Mother Boxes is now powering Cyborg, but the locations of the other two, i.e. the ones given to the Atlanteans and Amazonians, remain a mystery.
Warner Bros and DC have officially been quiet about Darkseid’s presence in the DCEU, but his existence was confirmed last year in Batman v Superman: Dawn of Justice not just with the deleted scene of Lex Luthor communicating with Steppenwolf, but through Batman’s Knightmare, which depicted a giant Omega symbol burned into the ground and Parademons flying about.