Batman/Superman #1 hits shelves August 14 and it appears we will get a Shazam Who Laughs in issue #1 or #2. He is on the cover art for issue 2. Put them on your preorders you never know.
From the description below, I’m gonna say #1 will be his first appearance. Might be considered a cameo as they’ll do a brief appearance on the last page or what not. But you can pre-order #1 now…
Batman Superman #1 (Yu Variant)
Up in the sky, in the dark of the night, trust no one-for the Secret Six walk among us. Spinning out of the devastating events of the Batman Who Laughs, Superman and Batman are together once more in an all-new monthly series-and they’re facing a terrifying new threat that could strike from anywhere. The Dark Knight and the Man of Steel must journey into the depths of Gotham City to learn which of their fellow heroes has been transformed into the horrifying horseman of their most dangerous and deranged foe ever. Our heroes will need to fight to survive, but an even more dangerous question lurks in the shadows: Can Superman and Batman even trust each other?
I say always go for first cover plus full appearance over first cameo.
And Cover A 
Only if it satisfies the first criteria…