USCIS Online Account Number is NOT a must-have in the immigration application process. It is usually a number offered to a person who has filed a certain application through the online filing system of the USCIS. To get a USCIS online account number,you will have to create a USCIS account in the first place.
If you want to find your USCIS online account number,you need to look at your notice of action letter. That is usually where it’s being featured. Going on your account settings will not reveal your number, which is why it can be a little confusing trying to find it. But if you receive a notice of action letter, you should look over it carefully and you should be able to find your online account number this way.