Your next step will be to give the seedlings gradually a greater exposure to sunlight. Once your first dragonfruit seeds have started to sprout to make the first dragonfruit seedlings, keep an eye on the tray until most if not all of the dragonfruit seeds have sprouted. You don't want to move the tray into a more sunny location too early (don't move the tray when the very first dragonfruit seedling pops up, wait).
When the tray is a little pasture of fresh new dragonfruit seedlings, now it is time to move the tray to give it more sunlight and at least a few hours of mild direct sunlight. The dragonfruit seedlings will be more hardy as they grown, and as their roots begin to penetrate the nutrient providing soil mix under the layer of the perlite win which you planted the seeds, the dragonfruit seedlings will start to get their moisture and nutrients from the soil mixture and be more tolerant of sun and surface drying. Start with just an hour or two of the weak sunlight of morning or evening, and gradually increase the amount of direct sun. Be extra careful not to let the seedlings get over-exposed to the sun at this stage, as they are still fragile, small cactus plants. They will likely just grow a little bit slower at worst if you increase sun slowly, whereas the worst case if you blast them with too much sun too soon is that they will be killed by it!
If the tray begins to get dried out, mist gently. Keep the tray from becoming too sodden, at this stage it is most likely better to err on the side of too little water rather than too much as you don't want the dragonfruit seedlings to rot.