Use the Company Share API to share updates and content to company followers on behalf of a particular company. The LinkedIn member making the call must be an admin of the company on behalf of whom content is being shared.
For the purposes of testing, we've created a special LinkedIn Company Page that is available for all developers to use. This company page doesn't require the authenticated user making the API call to be an administrator of the company: https://www.linkedin.com/company/devtestco (Company ID 2414183)
Note: Anyone can view the share when posted to this test company (since its purpose is as a developer sandbox for any developer using the API). Please ensure not to provide any information in the company share that you wouldn't want to be publicly viewable