You’ve got your wires a bit crossed here, Martin. FixedUpdate is an function that is called every time Unity updates the physics in your scene. Input.GetMouseButtonUp is an attribute that you can read to see if the mouse button was released this frame. Talking about putting a function within an attribute is meaningless.
What you probably want to do is:
void Update() //this is called every frame
{
if (Input.GetMouseButtonUp(0)) //the left mouse button was released
{
//do something
}
}