Can filler be used in the forehead to make it look rounder/less flat?

yes fillers can be used on the forehead but should be done conservatively because this area has many blood vessels and nerves.  Best, Dr. Emer.

Yes, fillers can be used to create a more rounded (less flat) forehead, although generally this is not considered a common treatment. Hyaluronic-acid based fillers are most commonly used since they look and feel natural, and there is an antidote enzyme (hyaluronidase) to dissolve it should there be any issues. Make sure to have only a highly trained and experienced physician injector such as a dermatologist treating you to optimize natural results and safety.

Fillers can be used to make the forehead more prominent and to fill in temporal hollowness. There are many choices of fillers for these regions. Some of the most common fillers used are Restylane Lyft, Juvederm Ultra and Juvederm Voluma. These fillers tend to create more lift and volume than others, which makes them the most appropriate products to use in this area.
Keep in mind that these are off-label uses of the product and you should have these areas injected by an experienced injector. Most of these fillers will last 9-12 months.