Aiming towards crosshair

I have a plane game where the view is from behind the plane, and you can control your movement of up, down, left and right with the WASD/arrow keys. I also have a crosshair that follows my mouse around the whole screen. I want bullets to spawn and fire towards my crosshair when I click the mouse, angles included. I know how to spawn the bullets and make them move, but I do not know how to make the bullets follow the crosshair (the crosshair is a 3D cube with an image on it).

You have the cube’s position and you fire the bullet forward.

bullet.transform.LookAt(cube.transform.position);