A double-faced spell is cast with its front face up by default. See rule 601, “Casting Spells.”
July 23, 2021 · Adventures in the Forgotten Realms
If an effect allows a player to cast a double-faced card as a face-down creature spell, or if a double-faced card enters the battlefield face down, it will have the characteristics given to it by the rule or effect that caused it to be face down. That card remains hidden, using a face-down substitute card (see rule 714) and/or opaque sleeves. See rule 708, “Face-Down Spells and Permanents.”
April 28, 2017 · Amonkhet
If an effect instructs a player to choose a card name, the player may choose the name of a front face of a meld card or the combined back face of a meld pair.
July 22, 2016 · Eldritch Moon
If an effect instructs a player to name a card, the player may name the front face of a meld card or the combined back face of a meld pair.