It’s called restarting the phone.

Eh? BFU ("before first unlock") is, by definition, the state that a phone is in when it is turned on. There's no need to "force" it.

If you mean forcing an iOS device out of BFU, that's impossible. The device's storage is encrypted using a key derived from the user's passcode. That key is only available once the user has unlocked the device once, using their passcode.