There isn't anything special you need to do on the account side. Just log in.
Per SE the requirements are:
Quote:
1. Your PlayOnline account is transferred to Square Enix account.
2. You must have an active PlayOnline/FINAL FANTASY XI service account.
3. Character status under the inactive character options must be set to "Cancelled" status as of
Monday, May 13, 2019 at 11:59 p.m. (PDT).
Which is to say, you need to have the basic SE account already (free), and associated character(s) that are inactive. During the free period, you can log on to them anyway.
What you're doing is how you'd reactivate your subscription, which is certainly what SE
hopes you'll end up doing...but it's not necessary for the event.