Creating a Texture Mask
Creating a Texture Mask is something that can be accomplished in any 2D image manipulation program. To accomplish this, all you have to do is paint the areas that you want to Mask with the color White leaving all the other areas Black. In the following image, we can see this in action.
*The source: Using Texture Masks in Unreal Engine | Unreal Engine 5.2 Documentation
To accomplish this, all you have to do is paint the areas that you want to Mask with the color black (and not with the color White).
Very special thanks to Epic for UE4