How to make assets generator to get canvas size when creating a png?

Hello. I'm trying to make assets generator to create a png with document size adding transparency to layer instead of streching.

Here's my document (width: 200px; height: 200px):

Then i name it "rounded_rectangle.png" and here's the result:

If i name it "200px x 200px rounded_rectangle_with_canvas.png" i get this result:

How can i acomplish this:

As u see the last one has the same size as document with the transparency included without streching. How can i make this with assets generator?

Hope i made myself clear.

Thanks in advance.