FFF is right. Just choose another username and password. If you read through the Help and FAQ you will also find that there is a list of flying sites which don't have ICAO indicators that you can also use for departure and arrival. These have been given dummy codes by AIS (there's a list on the site) but as the dummy codes do not start EG you will need to be registered for international use if you want to use them.
Regarding NotamPlot. AltAIS says the briefs are usable by NotamPro, does not mention NotamPlot.
There is a web utility called AIS2Plot but I have been unable to locate the URL, perhaps if someone else knows they can post. This is supposed to format the data for use in NotamPlot.
Please make sure that you fully understand the risks and limitations involved in processing the data this way.
Mike