What is the procedure to fill/send/approve a flight plan to the ATS network from outside?
Using EUROCONTROLs „business-to-business web services“ you can file and manage flight Plans (including IFPS validation) as well as check details about the filed flights e.g. departure and arrival messages or where the aircraft is currently.

After a registration (you have to proof you are eligible to use the service) you do not need any ATFM connection at all - you implement the API features of B2B services into your app and that‘s it.

For more details and the conditions check: https://www.eurocontrol.int/service/network-manager-business-business-b2b-web-services