Select New Flight and fill out all green highlighted boxes.
Click Generate Flight. This will create your OFP (operational flight plan).
In the OFP you can see important parameters such as air time, cruise FL, cost index, ZFW, and block fuel needed. In the top menu select Prefile on Network.
Select IVAO Prefile.
Review the generated flight plan on fpl.ivao.aero then click Submit FPL at the bottom.
You can now use the same callsign from the flightplan in Altitude to connect.