API wordpress install-plugin

I see the API supports installing a plugin to a wordpress instance.
- Do I need to put the plugin on the server before calling this? If so, where?
- How do I find out the asset-id for a plugin?

<packet>
<wp-instance>
<install-plugin>
<filter>
<id>WordPress instance ID</id>
</filter>
<asset-id>plugin ID</asset-id>
</install-plugin>
</wp-instance>
</packet>

Thanks
Dave