Problem adding reference on VS

Hello, i need to make an animation play before my gams changes the scene, for that i would like to use
yeld return new waitforseconds, however, for that i need the IEnumerator…witch i can’t find on my VS references (on add reference) how do i add it to VS?

Just add your timer IEnumerator script to any gameobject in the scene. And start it in void Start()