How do I find the angle between the players forward vector and the mouse position?

I’m trying to find the angle between where my player is facing and where my mouse is pointing.

If the mouse points in front of my character the angle should be 0. If it points behind the player is should be 180 or -180. To the right or left of the player it should be 90 or -90.

So in all, the angle needs to be relative to where the player is facing.

I was able to come across the answer with more searching. Here it is: