The official Japanese Movie 18 website has recently updated with information relating to the movie's pre-order ticket. Translations follow:

The special pre-order movie ticket giving access to a double giveaway will be on sale starting Saturday March 7th, 2015! If you purchase it at the theater you will be given a limited edition "Appear!" clear file folder!

The special pre-order movie ticket finally goes on sale Saturday March 7th!
This year we have a double giveaway! Amazingly, in addition to receiving the mythical Pokémon "Arceus", you will also receive one more legendary Pokémon! *
Look up further information on the "special pre-order ticket that includes an Arceus present code and an "Appear! Legendary Pokémon!" exchange voucher" this instant!
*The Pokémon can be received for the Nintendo 3DS games Pocket Monsters Omega Ruby/Alpha Sapphire/X/Y. The Pokémon are video game data, they are not figures, plushies, etc.

For further information, see the ticket section on the official Pokémon Movie Website

There's more! If you purchase the special pre-order movie ticket at the theater, you will receive an "Appear! clear file folder" limited to 300 000 patrons nationwide. It's a clear file with two different images: When you remove the sheet inside the folder, the legendary Pokémon will "Appear!"
*You will receive one folder with each special pre-order movie ticket purchased (parent-child tickets come with two folders).
*This folder is not for sale. Please do not purchase one from resellers.
*Each theater has a limited number of folders. Please understand if supplies have run out.
*Please contact your local theater for further information.

The mythical Pokémon "Arceus" will be available for download when the special pre-order movie tickets go on sale.

Look forward to Saturday March 7th, 2015!