Apple contacts export to LinkedIn

1. Open the Applications menu. ... 
2. Select Contacts.
3. Select the contact list you would like to export (e.g., All Contacts).
4. From the Contacts menu, select File, Export, then Export vCard... . ... 
5. Give your file a descriptive name, then select the location where you would like to save it.

The LinkedIn help pages state: *Supported formats include .csv, .txt, and .vcf.*

https://discussions.apple.com/thread/1557098

https://www.linkedin.com/help/linkedin/answer/46321/linkedin-integration-with-os-x-mojave-no-longer-available?lang=en