You have to call the prepaid customer service number at 1-888-294-6804 and ask them for the account number.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
Got to your "My Account" webpage from a PC. Right click and select "View page source". Scroll down to around line #194 and look for "accountNumber": "xxxxxxxxxx", That 10 digit number is your prepaid account number.