Just a nitpick, most savvy advertisers know that the discount code alone isn't how they measure ROI (or more appropriately ROAS) on a media buy. You look at other things like an increase in organic search or direct traffic, social mentions, etc. to try and paint a more holistic picture.
But they can attribute that directly to an ad spend. For instance the old joke is that the definition of a podcast is two guys talking about tech sponsored by SquareSpace.
I’m assuming that SquareSpace by knowing what percentage of the discount codes come from different podcasts can estimate where to spend thier money.
