string baseUrl = getJIRABaseUrl();
string requestUrl = baseUrl + "/rest/api/2/issue/" + key;
HttpRequest request;
HttpHeader authHeader = httpBasicAuthHeader("admin", "admin");
request.headers += authHeader;
request.headers += httpCreateHeader("Content-Type", "application/json");
string updateInfo = "{\"update\": {\"customfield_10001\":[{\"set\":\"null\"}]}}";
string resp = httpPut(requestUrl, request, updateInfo);
runnerLog(resp);