From Bulbanews, your community Pokémon newspaper.
To be sent out October 2
Customers who preorder Pokémon Ultra Sun or Ultra Moon from Amazon.es will receive a serial code to obtain Marshadow in Pokémon Sun and Moon. These codes are for the same Marshadow that was announced last month, when it was announced that codes to obtain Marshadow would be available at participating retailers in Spain and several other countries.
The code to obtain Marshadow will be sent via email starting October 2, 2017. The code may take up to 24 hours after placing an order to be sent. The code can be redeemed until February 23, 2018.
Furthermore, customers who pre-order from Amazon.es will receive a code to obtain 12 Quick Balls in Pokémon Ultra Sun and Ultra Moon, just like customers who purchase one of the games digitally. This code can be redeemed until November 30, 2018.
Marshadow will be level 50 and know the moves Spectral Thief, Close Combat, Force Palm and Shadow Ball. The announcement states that it will hold a Marshadium Z, but Marshadow is more likely to be bundled with the Marshadium Z, as is done with other event-exclusive Z-Crystals, since Z-Crystals that appear in the Bag to create held Z-Crystals cannot themselves be held.