code: public void addStaticAdditionalField(String keyValue) {
          String[] splitted = keyValue.split(":");
  
          if (splitted.length != 2) {
  
              throw new IllegalArgumentException("staticAdditionalField must be of the format key:value, where key is the "
                      + "additional field key (therefore should have a leading underscore), and value is a static string. " +
                      "e.g. _node_name:www013");
          }
  
          staticFields.put(splitted[0], splitted[1]);
      }
docs: Add a staticAdditional field. This is mainly here for compatibility with logback.xml
  
  @param keyValue This must be in format key:value where key is the additional field key, and value is a static
  string. e.g "_node_name:www013"
  
  @deprecated Use addStaticField instead