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

paid action
Your video creative asset can be incorporated into live broadcasts, with pre-roll, mid-roll, and post-roll available in various locations across Twitch.

You need this to get it done:

Video Specs (Ad Server Hosted)
Maximum video length: 30 seconds (unskippable) (can have up to 60 seconds in mid-roll with extra charge)
Minimum resolution: 1280 x 720 (1920 x 1080 recommended)
Minimum bitrate: 1000 Kbps
Peak Audio: -9dB
Video file format: .mp4 (h.264 encoded)
Maximum FPS (frames per second): 30
Companion Unit
Dimensions: 300 x 250 pixels
Max File Size: 50K
Max Animation: 15 seconds
File Format: .gif/.jpg/.png (click URL required)
Expandable: No
Third-Party Served (VAST/VPAID)
Video file requirements: (see previous video file specs)
Recommended video transcodes:
1920 x 1080 - 5,500 Kbps (required)
1280 x 720 - 3,100 kbps (required)
1024 x 576 - 1,750 kbps
888 x 500 - 1,316 kbps
854 x 480 - 1,200 kbps (required)
720 x 406 - 866 kbps
640 x 360 - 550 kbps (required)
480 x 270 - 384 kbps
320 x 180 - 200 kbps
284 x 160 - 180 kbps (required)
VAST/VPAID Version Support by platform:
Desktop: VAST 2.0/3.0 & VPAID 2.0
Mobile App: VAST 3.0
Console App: VAST 2.0
Must NOT have geo, browser or any other targeting on the third party end.
Third Party Tracking Events (Video)
Impressions, clicks, quartiles, completes.
HTML (Companion Unit)
Third Party HTML: Standard SSL Javascript tag only
SSL Compliance
Twitch requires all ads and third party vendors to be SSL compliant. All components of video and display ads including creative and tracking must be able to serve via SSL (https).
Requirements missing or found errors? Report

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

Twitch Advertising - Desktop Video
The official Twitch help page showing all specs and requirements for Desktop Video advertising.
Link missing here or found errors? Improve How-Tos