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

paid action
A conversational ad allows the user to respond to your promoted tweet by choosing one of the call-to-action hashtags to tweet (not available by default for self-serve advertisers.)

You need this to get it done:

A headline which appears above the hashtag for a one-choice hashtag option (maximum of 23 characters.)
Tweet Text
Text to accompany the image/video and hashtag button(s) (maximum of 280 characters.)
File format: .jpg/.png (non-animated .gif files are also supported)
Maximum file size: 3MB
Aspect ratio: 1.91:1
Image resolution: 800 x 418 pixels
File format: .mp4/.mov
Aspect ratio: 16:9
Maximum video length: 2 minutes and 20 seconds (however some advertisers are eligible to request an increase of up to 10 minutes)
Maximum file size: 1GB
Video codec: h.264 (high profile)
Audio codec: AAC-LC
Frame rate: 30fps (frames per second)
Minimum bitrate: 5,000Kbps
A hashtag for the user to choose (maximum of 21 characters.)
Pre-populated user Tweet
A pre-written tweet for the user (maximum of 256 characters.)
Thank You text
Appears after a user has tweeted your message (maximum of 23 characters.)
Thank You URL (optional)
URL to drive a user to after they have tweeted (maximum of 23 characters.)
Requirements missing or found errors? Report

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

Twitter Business - Advertiser creative specifications
Official information on creative specs for ads.
Twitter Business - Conversational Ads
Official information on Conversational Ads and how to set them up.
Link missing here or found errors? Improve How-Tos