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

paid action
The medium rectangle ad unit keeps viewers engaged as they scroll and explore content, and is displayed on the directory pages for games and channels.

You need this to get it done:

Banner File Specs
Dimensions: 300 x 250 pixels
Max File Size: 100KB
File Format: .gif/.jpg/.png
Expandable: No
Video: No
Max Animation Length: 15 seconds/3 loops (whichever is shorter)
Banner Tracking
Third Party Tracking: Impression, Click using iframe, Javascript, or Standard tags. No DMP or blocking tags. Daisy-chained or wrapped tags not preferred.
Targeting: Game Title, Game Genre
Initial load: 100KB
Polite load: 2MB
Third Party HTML: Standard Javascript tag or HTML5 zip file
SSL Compliance
Twitch requires all ads and 3rd 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 - Medium Rectangle
Twitch official guide for the Medium Rectangle.
Link missing here or found errors? Improve How-Tos