Subtract the projectile’s origin (or player’s origin) from the projected mouse position.
This gives you the “to” vector.
Then use that as follows:
You may be able to simply drive transform.forward
(or transform.up
or transform.right
when in 2D) equal to your movement vector.
Otherwise, use Mathf.Atan2()
to derive heading from cartesian coordinates:
Sine/Cosine/Atan2 (sin/cos/atan2) and rotational familiarity and conventions