For Season of Evil in Pokémon Go, there is an exclusive season-long special research project called Misunderstood Mischief that each player has access to for a limited time. Starting September 5 at 11am your local time zone, a variety of Misunderstood Mischief Special Research tasks will unlock, and upon completing them, you will have access to the capture of Hoopa in its confined form. It's a guaranteed encounter, but you should be able to catch it if you reach this stage.
Hoopa will not appear in nature as a five-star raid and will not hatch from an egg. It is a Mythical Pokémon, which means that it is only available at certain times in the game. Niantic is pretty good at making sure these Pokémon are generally accessible to all players, but never for too long. The activity becomes available when the Sunday event begins, the arrival of Hoopa.
You will only be able to catch Hoopa in this form once, so you can expect not to earn too much candy with it. If you want to power up Hoopa, you'll need to walk with it, slowly accumulating candy as you complete distance objectives or using rare candies. Although Hoopa is a Mythical Pokémon, it is not the best choice. We do not recommend doing this and investing resources in this Pokémon which we believe is purely a PokéDex filler. You should do this with other Pokémon.
The Season of Evil special research, Misunderstood Mischief, will only be available during this season. However, it does not need to be completed during Season of Evil. As long as you take the mission once, you will be able to complete it at any time, but it is advisable to do it sooner rather than later as some of the activities that may be released later will be easier on the days they are released.