Supplemental content can be determined through a server-side process, wherein a primary content provider can obtain the supplemental content from a supplemental content provider before any of the content is sent to the client device initially requesting the primary content. A primary content provider submits a request that can include an authorization token for the primary content provider, a publisher token including a proxy identifier for the user, application, or client device, and other relevant information. The supplemental content provider can verify the information and select targeted content based on the information. The supplemental content can then be provided to the primary content provider for incorporation with the primary content. The primary content provider can adjust the layout or selection before sending to the client device for display.