CanvasSpaceAnchoredPosition()

    public static Vector2 CanvasSpaceAnchoredPosition(RectTransform rectTransform, Rect canvasRect)
    {
        Vector2 pos = Camera.main.ScreenToViewportPoint(rectTransform.position);
        pos.x = (pos.x * canvasRect.width) - (canvasRect.width * 0.5f);
        pos.y = (pos.y * canvasRect.height) - (canvasRect.height * 0.5f);
        return pos;
    }

//  usage example.
//  rectTransform is a RectTransform  you want the canvas space anchoredPosition of,
//  but can't get because it's nested under other RectTransforms.
//  mainCanvas is the Canvas in screen space overlay render mode.

Vector2 anchorPos = CanvasSpaceAnchoredPosition(rectTransform, mainCanvas.rect);

// Now set the anchoredPosition of another RectTransform that you want to align to it to anchorPos.