<extensions>
    <add assembly="ISchemm.NLog.JsonTarget" />
</extensions>

<targets async="true">
    <target name="SampleTarget" xsi:type="JsonPost"
            url="http://myrestservice.com/logjson/">

      <field name="time" layout="${date:universalTime=True:format=yyyy-MM-ddTHH\:mm\:ss.FFFZ}" />
      <field name="msg" layout="${message}" />
      <field name="src" layout="${logger}" />
      <field name="lvl" layout="${level}" />
      <field name="exception" />                <!--The actual exception object serialized to json-->
      <field name="properties" /> 
      <field name="keyprop" property="a key"/>  <!--A value in the properties bag under key 'a key'-->
      <field name="another" property="5"/>  
    </target>
</targets>

<rules>
    <logger name="*" minlevel="Debug" writeTo="SampleTarget"/>
</rules>

var log = LogManager.GetLogger("myloggername");

var le = new LogEventInfo();
le.Level = LogLevel.Info;
le.TimeStamp = DateTime.UtcNow;
le.Message = "Hello, Json";
le.Properties["a key"] = "a value";
le.Properties[5] = 10;

log.Log(le);

*produces*

{ 
    "time" : "0001-01-01T05:00:00Z",
    "msg" : "Hello, Json",
    "lvl" : "Info",
    "src" : "myloggername",
    "properties" : { 
        "a key" : "a value",
        "5" : 10,
    },
    "keyprop" : "a key",
    "another" : 10
}