While you cant do it native (or I don’t know how) in Paint you can do very good round corner rectangles in mask mode and you can then add that to your mask /colour or what ever channel you want. You can correct roundness radius and animate it in Masks Tab.
OR you can draw your polygon of choice do a big blur on it and then levels compressing the range and you got your self round corners on anything you want.
here is Image of the workflow and I might even make it in to a small video tutorial 😉
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