When you receive a request to be a sender on a LinkedIn Sponsored Messaging campaign, you can approve or reject this request by following the link provided in the email. You can also manage Sponsored Messaging requests in Campaign Manager.
LinkedIn members may receive requests for Sponsored Messaging sender permissions from advertisers who would like to feature them as the sender of the Sponsored Messaging. If permission is granted, Sponsored Messaging can be displayed in the target audience’s LinkedIn Messaging and will appear to be sent from the designated senders. This allows for increased personalization and message relevance to targeted members.
Sponsored Messaging sender permissions are granted by account, not campaign. When you approve the sender permission for an ad account, you can be designated as a sender for future campaigns as well.
More detailed information on how to Manage Sponsored Messaging Senders
The video below describes in detail how to approve Sponsored Messaging requests in Campaign Manager