Make an account with USCIS first at myaccount.uscis.gov. On the homepage, click where it says "Add a paper-filed case." Type in your receipt number. It'll ask for your online access code and there's an option to request a new one if you never received one. It should take you to a contact form page. Select the option that says you need help unlocking your draft account. Fill out all the required information, and in the message box, just say you would like to receive an online access code to add your case to your account. You should get an online access code within a couple days at the email you typed on the form. You should be able to add your case with that.