If you found the citation in Articles+ or another article database, click the PennText button/link and use the Interlibrary Loan option from the PennText services menu to request it. This will automatically populate the request form with the citation information.

Otherwise, go to My Account in the catalog (upper right-hand corner of any catalog page) and log in using your PennKey ID and password as needed. Once logged in, select Request an item, then select Article or chapter, fill in the citation information and click the green Place Request button.