Yeah, I thought so. But still, it is not legal. Besides, you are basically asking people to spend money on your service, even if you are giving a discount. I think that the closest you can get to it is to target them with PPC, and perhaps even the Social ads, with a good selection of interests. Just make a discount offer and let them decide if they want to use the service or not. If you target them well, many will make a review afterwards, because that is what they do. If they don't want it that way, they surely won't want it by you approaching them and asking to use the service (and pay for it).