ZZZZ is a pseudo-code, used in flight plans for aerodromes with no ICAO code assigned.
How pilot actually file this flight plan? Is there anything special to do when file for 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.
The same wording is used for both FAA and ICAO flight plan instructions.