It appears there’s not a way to place objects over HTML forms with z-index (setDepth())?
All game objects are drawn on a canvas element, thus other DOM object will be above, or below them.
Add the following line to this example Phaser 3 Examples. You might also want to change the alpha in the div to 1 so it’s easier to tell which object is in front.
var image = this.add.image(400, 300, 'einstein');