You need to be on a visa/extension that requires it
You have to have filed ONE 90 day report in person,
go to this website,
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make a log in,
you can file a 90 day report online from 2 weeks before the due date down until 1 week BEFORE the due date (meaning you have a week window to file online)
You need to have a current TM30 and use the address you're registered at (the one you used for the previous in person 90 day report)
If you exit/re-enter the country the 90 day count down starts over and your next 90 day report is due 89 days from the date you stamped in
AND
sadly, you have to do that 90 day report in person as well π