You can make a selection with the Polygonal Lasso tool before applying the Curves tool. The rectangular selection marquee cannot be used here because the line between dark and light is not exactly horizontal.
Not asked, but can be useful:
You do not get the missing details back, only make the shimmering look out grey. You should recover the highlights starting from the RAW photo, if you have it. This can be done in a RAW developer, for example in Photoshop's Camera Raw.
If you have not the RAW, only the JPG, I recommend to use adjustment layers with masks. You can easily get different and re-adjustable treatments for different parts of the image. Here are few examples only for wakening up the interest, not claiming that these are fine by any measures. I've even not been there.