You can promote your content from your Organic Public Story, Saved Story, or Snap Spotlight content with ads on mobile, directly in the app.
To be able to promote content in-app, you must have a role on the Profile and be either an Account Admin, Campaign Manager, or Organisation/Business Admin on the connected ad account. You can learn more about roles and permissions here.
Go here to learn how to promote a Snap In-App.
Take advantage of all Snapchat has to offer. Promote your Snap to get more views and followers. Learn how to get started with Snap Promote.