When I start completing on-line form, I usually skip the point where they asked for postal or zip code and just leave it unavailable, blank (if not require), or just input a several digit(if require) without having an idea what is the correct value should be present in that place. That is not the problem of my location is not having postal code, but it is because I do not really know what it is referred to and what should I put in that field.
Finally, I found a resource where mostly of postal code in all sangkat in Phnom Penh City are defined. In case if you need it please feel free to get it as below. I know that recently that have lot of sangkat are migrated or separated from their original commune. I have tried to have all of them in here with Google maps pointing to where those sangkat located but some of them does not have yet the postal code for the new migration. They have had their old postal code but I preferred to leave it blank because it should not be used as they have been migrated already.