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

You need this to get it done:

A short description to accompany your ad (maximum of 90 characters.)
Target Audience
Decide on the audience you want your post to reach (gender, age, location, interests, etc.)
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.
The link you wish to include in your ad (where you would like to send your audience to.)
Ratio: 16:9 to 1.91:1
Maximum file size: 4GB
Minimum video length: 1 second
Maximum video length: 240 minutes
Captions: Not supported
Audio: Optional but recommended
Other recommendations: h.264 compression, square pixels, fixed frame rate, progressive scan, stereo AAC audio compression at 128kbps+.
A short headline to grab the audience's attention and briefly explain what your ad is about (maximum of 25 characters.)
Requirements missing or found errors? Report

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

Facebook Ads Help Centre - Messenger ads
Information about Messenger ads and how to set them up.
Facebook Ads Guide - Video (Messenger Inbox)
Specifications and requirements.
Link missing here or found errors? Improve How-Tos