Drone slack plugin: how to use recipient field

The drone slack plugin documentation states that instead of channel you could also use recipient, to send the notification to a specific user. How do you use that though? What is the Username?
I tried just providing my slack display name, but receive an channel_not_found error.

My config looks like this:

- name: slack
  image: plugins/slack
    webhook: https://hooks.slack.com/services/xxx
    recipient: MyDisplayName
    status: failure

I assume that MyDisplayName is wrong. The config works for posting to channels (just replacing recipient: … with channel: channelName