Displaying supplemental content in messaging interfaces

An online system displays supplemental content in conjunction with messaging threads between users and third parties to promote clear and productive conversations. The online system enables third parties to specify a set of content items to be displayed within a messaging interface when a messaging thread is initiated with the third party by other users. The set of content items may include one or more of images, videos, or web pages. When a user initiates a messaging thread with a third party, the online system accesses the set of specified content items and generates a messaging interface including one or more of the specified content items. By incorporating the content items within the messaging interface, the online system enables users and third parties to communicate while interacting with and viewing the content items.