Action Details
Requirements and how-to articles for Facebook's action:

paid action
Create a video ad to appear in between users' Stories.

You need this to get it done:

Video Specs
Ratio: 1.91:1 to 9:16
File format: .mp4/.mov recommended, but most video file formats supported.
Maximum file size: 4GB
Maximum video length: 15 seconds
Captions: Not available
Sound: Recommended
Objectives supported: All except Catalogue Sales, Store Visits, Messages
Write a short headline that grabs people's attention and tells them what the ad is about (maximum of 25 characters.)
A short description to accompany your video (maximum of 90 characters.)
Choose which type of call-to-action you want people to do when they see your ad.
Duration and Budget
Choose how long you want your ad to run and what your daily budget is.
Choose currency, enter payment details and pay.
Requirements missing or found errors? Report

How-To Articles
A list of useful links for you:

Facebook Ads Guide - Video (Facebook stories)
Specifications and requirements.
Facebook Stories Ads
Information about stories ads - when and how to use them.
Link missing here or found errors? Improve How-Tos