Board Thread:General Chit-Chat/@comment-27305952-20160329010710/@comment-27305952-20160410172057

"Unit of choice claiming form will be opened on April 26, 0:00 PST."

"Only players who completed the 30-day login requirement will be able to enter their Unit of Choice selection on the forum."

"Unit of Choice will be immediately sent to your Presents Box upon completion of the claiming form."

Grabbed straight from the News thread from the game. If you want more info, go read the News on the Omni Evo Login Campaign.