A dialog will appear asking if you want to apply or preserve the layer mask. Select preserve.

Now the two layers are merged and the layer mask is still there.

Step 17 – Import the image of the shattered glass
Choose File > Place, browse for the image of the shattered glass, then click OK. Press enter on your keyboard to exit out of the transform tool.

Step 18 – Add transparency to the glass layer
Using the same technique used earlier, we’ll make the dark area of the layer transparent so that it blends into the photo. Right click on the layer with the glass then choose Blending Options.

At the bottom of the window in the blend if options, drag the black input sliders towards the right until the background disappears.

Hold the Alt key on your keyboard then drag the right black input slider towards the right until the edges are softened.

Change the blending mode of the current layer to luminosity. This will allow the color from the layers below to be seen through the glass.
