If there are no existing mounts on the gun, you do have some options.
1.) Drill the holes and tap it yourself. If you have the know how, equipment, and the will to do it. Tapping the forend's can be tricky depending on the mounting hardware you want to use and the thickness of the forearm, you may have to cut/grind the screw down.
2.) There are some slings that don't use mounting hardware, either the strap goes around your stock, or has like a slip on recoil pad with the strap built on to that. I think the majority of these slings are meant just for carrying your gun to the field/blind, as they will change your gun mount a little, and usually the front of the sling slips over the top of the barrels, which can obscure your sighting.
3.) The post above has the barrel band mount. That is probably the easiest option for the front of the gun, but again, you may need to drill or figure out something for the stock end of your gun. Also, it may leave rub marks on your barrel, so beware of that.
One of the issues if trying to tap your forend is, the length from the stock to there could be a little short for a sling. So you might have to get creative building an extension or a barrel band type of thing that won't leave rub marks.