An online system enables developers of third-party applications to specify online system users to receive advertisements. The online system generates and sends a user-specific identifier for online system users that use a third-party application when an advertiser associated with the third-party application requests a user-specific identifier from the online system. For example, the online system retrieves user identifying information associated with a client device identifier provided by an advertiser and generates a user identifier by encrypting the retrieved user identifying information. The advertiser may include one or more user-specific identifiers in an advertisement request sent to an online system to present an advertisement in the advertisement request to online system users corresponding to the user-specific identifiers. The online system determines whether to present the advertisement to users based in part on whether users correspond to user-specific identifiers included in the advertisement request.