code: protected final void addAdditionalParameters(List<ContentPart<?>> contentList)
      {
          //add additional parameters
          if(this.additionalParameters!=null)
          {
              Iterator<Entry<String,String>> iterator=this.additionalParameters.entrySet().iterator();
              Entry<String,String> entry=null;
              String key=null;
              String value=null;
              while(iterator.hasNext())
              {
                  //get next entry
                  entry=iterator.next();
                  
                  //get next key/value
                  key=entry.getKey();
                  value=entry.getValue();
                  
                  if((key!=null)&&(value!=null))
                  {
                      this.addContentPart(contentList,key,value);
                  }
              }
          }
      }
docs: This function adds the additional parameters.
  
  @param   contentList
  The content list with all the created parts