code: public T extractFromHeader(String headerName, String variable) {
          if (headerExtractor == null) {
              headerExtractor = new MessageHeaderVariableExtractor();
  
              getAction().getVariableExtractors().add(headerExtractor);
          }
          
          headerExtractor.getHeaderMappings().put(headerName, variable);
          return self;
      }
docs: Extract message header entry as variable.
  @param headerName
  @param variable
  @return