[SerializeField] private AnimancerComponent _Animancer;
[SerializeField] private AnimationClip _Clip;

private void Awake()
{
    // Start fading the animation normally.
    var state = _Animancer.Play(_Clip, 0.25f);
    
    // Then apply the custom fade to modify it.
    CustomFade.Apply(state, Easing.Sine.InOut);// Use a delegate.
    CustomFade.Apply(state, Easing.Function.SineInOut);// Or use the Function enum.
    
    // Or apply it to whatever the current state happens to be.
    CustomFade.Apply(_Animancer, Easing.Sine.InOut);
    
    // Anything else you play after that will automatically cancel the custom fade.
}