The Atlassian draw.io plugins are pay via Atlassian products, this means all billing and licensing is direct with Atlassian, we do not have access to billing and licensing information.
If you are unable to locate licensing information for a purchase, first check your email (including spam folder) around the date and time of purchase, looking for emails from Atlassian.
If nothing is found, go to https://my.atlassian.com/products and log in with the email used to perform the purchase. The billing details and license key will be displayed when you expand the section containing the draw.io purchase.
If you do not know the email used, file a support ticket with Atlassian at https://www.atlassian.com/company/contact/purchasing-licensing. Click "Payments and refunds" then "request a copy of your invoice" and fill in all available details you can to assist Atlassian in finding the order.