code: protected void setPayload(String payload) {
          MessageContentBuilder messageContentBuilder = getMessageContentBuilder();
  
          if (messageContentBuilder instanceof PayloadTemplateMessageBuilder) {
              ((PayloadTemplateMessageBuilder) messageContentBuilder).setPayloadData(payload);
          } else if (messageContentBuilder instanceof StaticMessageContentBuilder) {
              ((StaticMessageContentBuilder) messageContentBuilder).getMessage().setPayload(payload);
          } else {
              throw new CitrusRuntimeException("Unable to set payload on message builder type: " + messageContentBuilder.getClass());
          }
      }
docs: Sets the payload data on the message builder implementation.
  @param payload
  @return