Vertical scar on forehead
There are multiple ways to improve the appearance of scars. Fillers are one way to improve the look of indented scars. From the picture, it appears that you could benefit from this approach. I recommend that you see a facial plastic surgeon who can evaluate you and discuss all your options.
There are several different ways to improve a scar, with fillers beings one of them. I do think filler could be helpful, as would microneedling.