All aircraft entering U.S. domestic airspace from points outside must provide for identification prior to entry or exit in and out of an Air Defense Identification Zone (ADIZ). ADIZs have been established to assist in early identification of aircraft in the vicinity of international U.S. airspace boundaries (AIM Section 6, 5-6-1). There is no ADIZ between the U.S. and Canada.
To do this you must file a Defense VFR flight plan (DVFR). To File a DVFR flight plan, you will need to use the ICAO flight plan form and enter your ADIZ penetration time and location in the REMARKS section on the ICAO form (Equivalent to line 18 - Other Information on the printed ICAO form). ForeFlight does this automatically if you file DVFR.
Here are the steps to file a DVFR flight plan in ForeFlight.
Calculate a VFR flight plan from either the Map or the Flight page. Select Proceed to file, then change the “Flight Rules" to DVFR. Then file the flight plan.
You will be able to see the EET to the ADIZ (or Flight information region) by clicking on the navlog. For Defense VFR (DVFR) flights, the estimated time of ADIZ penetration must be filed with the appropriate aeronautical facility at least 15 minutes before penetration, except for flights in the Alaskan ADIZ, in which case, report prior to penetration. Additionally, VFR pilots must receive and transmit a discrete transponder code.
According to FAR Part 99, if penetrating an ADIZ, all aircraft of U.S. or foreign registry must file, activate, and close a flight plan with the appropriate aeronautical facility. In addition to normal ADIZ position reports, and any other reports Air Traffic Control may require, a foreign civil aircraft must give a position report at least one hour before ADIZ penetration, if it is not more than two hours average cruising speed from the U.S.