To get your hands on the Pokémon you will have to get a serial code from a participating store. In the UK and Germany you'll need to go to Smyths Toy Store while in France you'll have to make your way to PicWicToys. Details of the Mewtwo are as follows:
Held Item: None
It has also been confirmed that the Mewtwo will have maxed AVs, so you don't need to use candies to increase its power. The event will begin on the 9th of August and run until the 6th of September, so you have a whole month to claim this legendary Pokémon for yourself!
The event has not yet been confirmed outside of Europe or the UK, but we'll keep you updated if that changes.