A person can pre-register to vote using any of the means available for registering to vote. A person who has pre-registered to vote is automatically deemed registered to vote on their 18th birthday unless the person's pre-registration is canceled.
When a 17-year-old preregisters to vote they will receive a letter from their county elections department in the mail advising that their application was received but they will not become an active voter until their 18th birthday.
Determining if applicants can pre-register and when they will be able to vote:
https://www.calculator.net/age-calculator.html