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Unfortunately I'm on 8105m. Not certain if the HTML canvas is still relatively the same in 8.2.03.
If you click on the button in the HTML canvas (I'm assuming you're using the stock button from the components tab) go to the properties and scroll until you find the Border section. Click to expand and scroll again down to the border-radius. Enter a value in pixels, so maybe 5px or 10px. That will give you the rounded edges you might be seeking.