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

Share your thoughts and ideas with other LinkedIn users by writing a post.

You need this to get it done:

The content of your post (maximum of 1,300 characters.)
Posting Location
If you are an admin for a company page(s) you can choose whether to post from your personal profile or from a company page.
You can choose who your post is visible to - Anyone, Connections only or Group members. You can also choose whether to enable or disable comments on your post.
Hashtags (optional)
You can add hashtags to your post in order to help it get discovered by other users. However, be careful not to add too many (the optimum number is generally believed to be between 3 and 5) as it could risk looking like a spam post.
Requirements missing or found errors? Report

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

Posting and Sharing Content - Overview | LinkedIn Help
How to write a post and share it with fellow LinkedIn users.
Link missing here or found errors? Improve How-Tos