Assuming your projectile object uses the bullet behavior, you need to modify the angle of motion of the bullet on spawn, accordingly to the angle of your shooter object.
You should likely use the Set angle of motion action when your object is spawned.
Projectile. Set angle of motion to Shooter.angle
Would set the angle the Projectile is moving to the same angle Shooter is looking at.
That would be for a top down view project.
On a platformer where the Character sprite will likely be mirrored, you should likely use sub-events, to check whether Character is mirrored or not and set the angle of motion of Projectile to either 0 (going right) or 180 (going left).
It depends on your project in the end.
You might want to post your .c3p file in order for us to provide an accurate answer.