To unlock the scene, you need to at least be at 500 desire (half the bar) with Mayu, gift the outfit to her and ask her to take the pose when this is offered.
Hi, the outfit was implemented in the v0.07, the latest update available on subscribestar. From there, you have to buy the outfit in the shop, gift it to Mayu by clicking en "gift" when interacting with her and if you have at least 500 desire (half the bar), you can unlock the h-scene.