I support @Albrecht's answer but I will try to explain it slightly differently.
Sit at the keyboard at a good height. (Don't play standing up.) If you are in a regular straight-backed chair, you'll probably need to raise it with, for example, a couple of fat phone directories. If you use cushions or pillows, try to choose flat ones, and avoid the rounded kind.
Start by playing the octave comfortably, that is, the low A# and the higher A#, thumb and pinkie. There should be a slight curve in your fingers when you play keyboards. The curve is the type of curve that you would have if you were picking up a glass of water.
Trim your fingernails, so that you can comfortably play neighboring keys with this type of curvature. The fleshy part of your fingertip should be able to come into contact with the keys, without any click sound of a fingernail hitting the key.
Practice playing the alternating A sharps, sometimes as a chord and sometimes as a broken chord (that is, a sequence of notes).
Think of the key on a piano extending into the guts of the piano as a long lever arm. To get maximum leverage with a black note, it's helpful to have your finger in contact with the black key at the very edge of the key (the edge nearest your body).
Try to avoid having unnecessary tension in your fingers, hand, wrist or arm. Ask someone to look at you and let you know if it looks like you are hunching or raising your shoulders at all. If so, let your shoulder relax. If the person notices any clenching in your hand or anywhere else, they should let you know that as well, so you can tell that part of your body that it can relax.
To encourage a relaxed position, it can be helpful to allow a subtle rolling back and forth as you move from pinkie to thumb, to pinkie, to thumb. (I'm trying to describe a slight rotation of the arm.)
To add the F, notice that your index finger will hit the white key in approximately the middle of the key (that is, the middle along the length of the key), and it will be more curved than the pinkie and thumb.
Let us know how this feels. First, work on getting a good hand shape, body position, etc., before you work on your volume.