Place the 15A breaker in the box in the slot below the second breaker.

Stop right there.

Why would you even consider plugging in the breaker to the box before
connecting the black wire to it?

If you cannot think of a safer way of doing this then please consult a
professional before you hurt yourself.

Don't take this as a personal attack, just what you described is
ill-advised and sounds like your first attempt at residental electrical
wiring. Be safe.