Custom Script and Controls for Canvas UI button ??

I CAN NOT USE IENUMERATOR and yield WITH CANVAS BUTTON !!!!!!

the thing is play button (canvas UI) calls a function 'changeScene(string nexttScene) and passes a string ( name of next scene ) to the gameManager object which is set by onClick thing of canvas UI.

Now since there is a IEnumerator in the changeScene function which calls a fader script to fadeIn and fadeOut the scene and yields for 3 seconds. but the scene changes Instantly.

how can i not use the default onClick and apply some custom script to canvas Button

please write the requireComponent, Getcomponent part and bring the control of button to the script. rest i can do.