How to file flight plan with ZZZZ?
I don't know if you're asking about a specific country, but in the US it seems that you can just use it in place of a regular airport identifier and add supporting information in the remarks.

Specifically, the AIM 5-1 has several references to it, including:


  (b) If no four-letter location indicator has been assigned to the
  departure aerodrome, insert ZZZZ and specify the non-ICAO location
  identifier, or fix/radial/distance from a nearby navaid, followed by
  the name of the aerodrome, in Item 18, following characters DEP/,
  
  (c) Then, without a space, insert the estimated off-block time.
  EXAMPLE-
  
  
  KSMF2215
  ZZZZ0330
  


The same wording is used for both FAA and ICAO flight plan instructions.