Is it safe to mount a TV below a window?
Your two options are to attach the TV mount to one stud (try to hit the center of the stud) or mount a board to the wall that hits more than one stud and then attach the TV mount to that (or get a TV mount made to pick up more than one stud).

For larger TV's I would want to hit at least two studs.  I looked up a random 43" TV on Amazon and it's weight was listed as 17lbs.  Unless it's a very old flat TV, it's going to be pretty light, and one stud with a strong mount should be fine.

Use a stud finder to get a general idea of where the stud is.  Then take a small drill bit (1/16" is fine) and drill a series of test holes to find the exact center of the stud.



Picture from another question

Your window is higher, but the framing will be very similar.  The cripple stud under the window will be securely nailed into the sill plate.  That and the strength of the drywall will keep it from pulling out or moving.  When installed properly, studs and drywall are very strong.