To use offline conversions with LinkedIn, you need to go into settings ⇒ integrations ⇒ LinkedIn ads ⇒ configure:

Then you need to choose an ad account, the action you want to send as a conversion, and the attribution-level.

After you configure the conversion goal, it takes 24 hours to start sending the data.

At any point, you can see the list of your offline conversions on the same page: