Can you sideboard in a Best Out of 1 Pre Rift event?

No, you cannot sideboard in a Best of 1 Pre-Rift event — by default.

In Riftbound, sideboarding is only permitted between games of a match (Rule 403.4). Since a Best of 1 match has only one game, there are no "between games" opportunities, and Rule 403.5 explicitly states players may not use sideboard cards in the first game of a match.

That said, there is one exception: the head judge may opt to use an altered setup process for Best of 1 that includes a sideboarding step during initial setup (Rule 406.1.g.5). If the judge has announced this change, sideboarding would be allowed before game 1 starts. But for a standard Pre-Rift event without that specific announcement, the default rules apply — no sideboarding.