Go to mpmissions\dayzOffline.<mapname>\expansion\settings in your server directory and open the file basebuildingsettings.json

{
    "Name": "My Location Name",
    "Center": [
        11882.0,
        143.0,
        12466.0
    ],
    "Radius": 1000.0,
    "Items": [
        "MyClassName",
        "MyOtherClassName",
        "MyLastClassName_WithoutACommaAtTheEnd"
    ],
    "IsWhitelist": 1,
    "CustomMessage": "If you want a custom message to display to the player"
},

How does it work ?

{
    "Name": "My Location Name",
    "Center": [
        X, <= West/East
        Y, <= Elevation/Altitude
        Z  <= South/North
    ],
    "Radius": 1000.0,
    "Items": [
        "MyClassName",
        "MyOtherClassName",
        "MyLastClassName_WithoutACommaAtTheEnd" <= Without a comma if it's the last item of the list
    ],
    "IsWhitelist": 1,
    "CustomMessage": "If you want a custom message to display to the player"
} <= Without a comma if it's the last zone

• Name: The name of your zone, it will be displayed as a notification Title if the player try to build with an item not allowed to be used in this area.
• Center: The position of your zone
• Radius: The size of your zone
• Items: A list of classnames (items)
• IsWhitelist: If set to 1, only the Items will be allowed to be used in this zone. If set to 0, all the Items will be disallowed in this area. Transforming the Whitelist into a Blacklist.
• CustomMessage: Similar than "Name", this message will be displayed bellow the title

The easiest method we recommend you to do is to go at the location you want to add your safezone and open your admin tool to get this coordinates.

Think to remove the , in }, at the last bracket.

As you have noticed, this file also contain various settings. If you are curious about any of them, we recommend you to read the server settings documentation of this file ( https://github.com/salutesh/DayZ-Expansion-Scripts/wiki/%5BServer-Hosting%5D-SafeZoneSettings )