Sorry, my mistake - you can't do it with Paster, but it's possible with Canvas. (You are using Construct 2, right?)
It's not an easy task - you will need to draw a path with 4 arcs and 4 lines between them.
I have a demo, but it's for C3 and there I used circled for corners:
Hey, thanks for that! I'm using C2 unfortunately. But, I just followed your initial advice, which make without Canvas/Paster plugin and just make a rounded-corner on Figma or something and then import it to Construct 2 using 9-Patch. I just made multiple rounded corner :D. I realized I do not need dynamic corner radius. Thanks though!
And I also found out Canvas plugin make the device slow? I'm planning to release my game into mobile. So that's also half a reason on why I don't want to use that plugin :p. If it's not that slow, I will though!