An online system provides an advertisement (“ad”) request including ad content and a prompt to perform an action presented in association with the ad content. For example, the ad content is video data and the prompt for performing an action is presented after the video data is presented or after a portion of the video data is presented. An advertiser may include a custom prompt in the ad request that includes text information and instructions associated with the text information that perform an action when executed. Multiple prompts may be included in the ad request, and the online system selects a prompt based on information maintained by the online system and associated with a user to be presented with the ad content. The selected prompt is communicated to a client device for presentation in association with the ad content.