When setting up your company LinkedIn Page you are prompted for your Company Page ID here:
Note: If you are posting to a LinkedIn company page, put the id of the company here. You must be an admin of that company on LinkedIn. Leave this blank to post to your own LinkedIn account.
If you do not see a numeric ID you must contact your Linkedin Company page administrator for the numeric ID.
To find your company ID go to your LinkedIn Company page and your ID is the last section of the url. Using ApplicantStack's company page as an example:
Therefore ApplicantStack's company ID is 674969.