I have a big line dent on my forehead. How can I fix it with medicine or something? (Photo)

You need to have an experienced injector use a filler that is temporary to slowly fill the dent. I would not use fat for this as you can have irregularities that will show. Voluma will last 2 years and and is reversible.

You have a central forehead concavity due to the shape of the bone. While injectable fillers can be initially used, a permanent solution is to build up the lower forehead areas with bone cement.

I would use something like juvederm or restylane here. These are more reliable than fat and in the forehead these products stick around  for 2-4 years.