code: public Map<String,String> getRequestHeaders() {
  
          // If we already set the headers when logging request metadata, use them
          if (super.getRequestHeaders() != null)
              return super.getRequestHeaders();
  
          try {
              Map<String,String> headers = null;
              String headersVar = getAttributeValueSmart(HEADERS_VARIABLE);
              if (headersVar != null) {
                  Process processVO = getProcessDefinition();
                  Variable variableVO = processVO.getVariable(headersVar);
                  if (variableVO == null)
                      throw new ActivityException("Headers variable '" + headersVar + "' is not defined for process " + processVO.getLabel());
                  if (!variableVO.getType().startsWith("java.util.Map"))
                      throw new ActivityException("Headers variable '" + headersVar + "' must be of type java.util.Map");
                  Object headersObj = getVariableValue(headersVar);
                  if (headersObj != null) {
                      headers = new HashMap<>();
                      for (Object key : ((Map<?,?>)headersObj).keySet()) {
                          headers.put(key.toString(), ((Map<?,?>)headersObj).get(key).toString());
                      }
                  }
              }
              Object authProvider = getAuthProvider();
              if (authProvider instanceof AuthTokenProvider) {
                  if (headers == null)
                      headers = new HashMap<>();
                  URL endpoint = new URL(getEndpointUri());
                  String user = getAttribute(AUTH_USER);
                  String password = getAttribute(AUTH_PASSWORD);
                  String appId = getAttribute(AUTH_APP_ID);
                  if (appId != null && appId.length() > 0) {
                      Map<String,String> options = new HashMap<>();
                      options.put("appId", appId);
                      ((AuthTokenProvider) authProvider).setOptions(options);
                  }
                  String token = new String(((AuthTokenProvider)authProvider).getToken(endpoint, user, password));
                  headers.put("Authorization", "Bearer " + token);
              }
              // Set the headers so that we don't try and set them next time this method gets called
              super.setRequestHeaders(headers);
              return headers;
          }
          catch (Exception ex) {
              logger.severeException(ex.getMessage(), ex);
              return null;
          }
      }
docs: Override to specify HTTP request headers.