Can I sideboard in game 1 before the game starts?

No, you cannot sideboard in the first game of a match.

According to Rule 403.5 of the Tournament Rules: "Players may not use sideboard cards in the first game of a match." Additionally, per Rule 406.1.e, the start-of-game procedure—which includes placing your Legend and Chosen Champion—occurs as part of the initial setup, and you are not permitted to use sideboard cards at this stage. Sideboarding is only permitted between games of a match (Rule 403.4).