How do I create round edged squares or rectangles?

I saw that 9-patch plugin can be helpful in this. But I am not sure how to do it?

I have already checked this topic and want something like that.

https://www.construct.net/en/forum/construct-2/how-do-i-18/border-radius-sprite9-patch-151558

I have already checked this topic and want something like that.

https://www.construct.net/en/forum/construct-2/how-do-i-18/border-radius-sprite9-patch-151558

So what exactly is the problem? There are screenshots and links in that post.

There it shows the completed version in the screenshot. However, I am not sure how those round edges were created. How do I specify the radius for the edges?

You need to create the image in some other editor - Paint, Paint.Net, Inkscape etc.

9-patch will allow you to resize the rectangle, but the corner radius will remain the same.