You have to just keep checking the page source every day to see it appear in the 'vin' field. It's called the hidden VIN because it isn't on the front-end 'visual' render of your account page, it's in the HTML code of the page.
Easiest way to find it is to log into your Tesla account, open a new tab and then use this URL:
https://www.tesla.com/teslaaccount/oxp-bff-api/user-orders
if the 'vin' field says 'null' or just quotation marks, you haven't got one assigned yet, otherwise you will see your VIN there.