I don't get what you mean, but if you are using physics (which I assume, as you speak of adding force), you would want to use "Set force toward position" and use MouseX & MouseY coordinates.
Okay, let me see if I got it right...
1. Shoot a bullet
2. Bullet collides with box and applies force to it
3. Box reacts to the force
In that case, you want an on collision event with "set force" action on box:
X component = ForceAmount*cos(AngleBetweenBulletandBox)
Y component = ForceAmount*sin(AngleBetweenBulletandBox)
ForceAmount could be the bullet's X or Y component of the speed.