It's usually a random draw, but some of the sportsbook apps actually offer these now with the opportunity to pick.

Unfortunately, of course, they assign different payout odds to each square, which would be less (or a lot less) for the more common number pairs. And they do it in a way, mathematically, whereby they can pretty much guarantee that they make money.

For example, if a square with a 3.0% probability were fairly priced, it would be priced at +3233 (i.e., risk $100 to make $3,233). They'd probably price that square around +2900 instead, essentially creating negative expected value for bettors (who run the math) in every single square.