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

paid action
Sponsored InMail allows you to drive more leads and engage your target audience by delivering personalised, private messages right to their LinkedIn inboxes.

You need this to get it done:

Campaign Manager Account
In order to be able to use LinkedIn's Campaign Manager, you must made an account for it. You need to enter an account name (up to 100 characters including spaces), your currency (which cannot be changed later) and if you wish you can associate a LinkedIn Page with your account, but this is optional (although some ad formats require a Company or Showcase Page to be linked.)
Campaign Name
The internal name for your campaign (not visible to the public.)
Sponsored InMail Ad Name
A name for your Sponsored InMail Ad.
Message Subject
The subject line for your InMail message (maximum of 60 characters.)
Message Text
The main body text for your InMail message (recommended to keep it under 1,000 characters.) You can use custom fields for the user's first and last names.
Landing Page URL
The URL which the user is taken to when they click the call-to-action button. (must begin with http:// or https://)
Call to Action Button Text
The text to be displayed on your call-to-action button (maximum of 20 characters.)
Target Audience
Choose your target audience based on attributes such as location, job experience, interests, etc.
Choose the language of your campaign.
Banner (optional)
A banner image to be displayed to the right of the InMail message.
File format: .jpeg/.jpg/.png
Resolution: 300 x 250 pixels
Decide how long you want your campaign to run. A starting date is required (either start immediately or schedule a starting date) and you can enter an end date if you wish, or leave it blank for your campaign to run until your budget runs out.
Budget and Bid Type
Choose your budget (either daily or total) and the bid amount. The bid type for Sponsored InMail is cost per send.
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How-To Articles
A list of useful links for you:

LinkedIn Help - Sponsored InMail Overview
Official information on Sponsored InMail.
Link missing here or found errors? Improve How-Tos