If you are using a tag-based "smart playlist", you just need to add the appropriate tag to the video.
{"method": "create_video", "params": {"video": {"name": "Name","tags": ["playlistTag"],"shortDescription": "...","itemState": ""},"token": "TOKEN","encode_to":"","create_multiple_renditions": ""}}
If the video is to be added to a manual playlist, use update_playlist after create_video.