'How to know when user click on skip button in vast

I'm using vast version 3.0 and I want to know when user hits the skip button (after n seconds after ad played). I read vast document https://www.iab.com/wp-content/uploads/2015/06/VASTv3_0.pdf but I couldn't find the answer

Here is part of my vast which only can receive skip event (without knowing n):

<VAST version="3.0">
  <Ad id="123">
    <Inline>
      <Creatives>
        <Creative>
          <Linear skipoffset="00:00:05">
            <Duration>00:00:06</Duration>
            <TrackingEvents>
              <Tracking event="skip">
                Link
              </Tracking>
            </TrackingEvents>
          </Linear>
        </Creative>
      </Creatives>
    </Inline>
  </Ad>
</VAST>


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