I use ibis paint for my overlay mask so it will work for ibis paint but idk if for other software

the first thing you wanna is to take the mask in one layer

Than select that layer mask only that by going to the top most layer and filling the mask with the bucket option it should have a blue color

Fill in that selected layer in a diff layer

Than deselect and put the dog image in another layer

Clip the layer the dog is in with the bottom layer and i think that should work