The online account number is a number provided by USCIS for you to be able to login and manage your application. This online account number is not something everyone has; you may only have this if you applied for one in the past.
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If you don't have this, please leave the box blank. USCIS will assign you an account number and send it in mail to your registered mailing address you provided for you to be able to set it up once you have submitted your petition.