There isn't a function that will do this for you in Studio, you will have to place the snowflakes yourself.

I would do one line, then duplicate and rotate to get a second side.
Line up the second line with edge of the first.
Weld if necessary eg. overlapping the end snowflake.
Select all and group
Duplicate and rotate and position to make the square
Select all and group again
Now apply Offset path to get your outline.

If there any bits inside the outline, ungroup or release compound path, you may have to do both depending how you make it.

Now drag the out line away and your square of snowflakes, select all the bits and delete, now you have what you want a square shaped mat, with snowflake cut out's around the edge.