I’m 18 and I want a small hair transplant to fill in the area shown in the photo

At 18, if you have a hair transplant, you will almost certainly accelerate the loss that you see and get a much larger area. Then you will transplant it again, and more hair loss will follow with then a 3rd transplant and on and on. At your age, the best treatment is the drug finasteride which may very well reverse this without surgery and slow down the development of more hair loss. Doesn’t that make more sense?