public class WaitForMouseDown : CustomYieldInstruction
{
    public override bool keepWaiting
    {
        get
        {
            // Example of condition :!Input.GetMouseButtonDown(1);
            // To keep coroutine suspended, return true.
            // To let coroutine proceed with execution, return false.
            return false;
        }
    }
    
    // Not sure about if this constructor is necessary.
    public WaitForMouseDown()
    {
        Debug.Log("Waiting for Mouse right button down");
    }
}

public class GameManager{

    public void Start(){
    
    var routine = waitForMouseDown();
    StartCoroutine(routine);
    }
    
     public IEnumerator waitForMouseDown()
    {
        yield return new WaitForMouseDown();
        Debug.Log("Right mouse button pressed");
    }

}