You can receive your Egg by choosing Get via Internet from the Mystery Gift function in the game until February 21, 2022. By putting it into your party and carrying it around on your adventure through the Sinnoh region, you’ll be able to hatch the Egg and gain Manaphy as a companion.
Source
It was given in Pokemon Ranger: Guardian Signs as an Egg you transfer to a Gen 4 game from 10/4/2010 to 1/11/2011.
