How can I stretch polygon uniformly to rectangle?

I'm assuming you are using CorelDRAW? In the future, add your post to the forum that matches the version you are using -- or at least state which version you are using.

If you want it to fill the width of the page with your shape, first center the object on the page. Then type in the width of the page in the Object size box on the Property bar, make sure the Lock aspect ratio is ON. Press Enter to see the change.

Another way:  Enable snap to grid. Select the shape and then while holding down the SHIFT key, drag one of the corner handles to resize as needed.