Specific License number for specific car model?

Alright, straight to it. I want a specific vehicle model, to ALWAYS spawn with a specific license plate, and plate number.

The license plate was easy...

However, I can't figure a way to make it always spawn with a specific plate number.

Here is what I have: 

<plateProbabilities>
        <Probabilities>
          <Item>
            <Name>Police guv plate</Name>
            <Value value="100"/>
          </Item>
        </Probabilities>
      </plateProbabilities>

This is for "police", and I want to make the plate number to read "N 622".

Why do I want to do this? This is for a server I run, and I want the NPC police vehicles to spawn with that. We use "N 622" as a code to determine it is someone impersonating an officer in our server.