Please follow the next steps in order to find the ID:
1) As a company page administrator, your Company ID can be retrieved by navigating to the admin section of your company page. For example, the LinkedIn Company Admin Page is https://www.linkedin.com/company/1337/admin/. The Company ID is 1337 in our example.
2) Copy the ID and paste it in "Company or Showcase Page ID" field.
3) Save changes.