To answer your question it would be important to understand the nature of your "dent", forehead depression. For example was it from trauma with soft tissue atrophy, were you born that way (congenital), is it due to a bony depression in your skull. A CT scan and certainly an exam would be helpful.
In my experience though fat transfer might work, a more predictable solution would be with a long term filler such as Radiesse. If a bony deformity then "bone paste" or even acrylic, or permanent filler can be placed. Alternatively, Alloderm or other even a dermal-fat graft (which can be taken from an existing scar might be used to fill this in. My advise is to have a consult and work up by a board certified plastic surgeon and discuss in detail your options