/* @方法1 */
String kPath

kPath = "/some_static_code_here/plus?";
kPath += macstr;
char kURL[50];
kPath.toCharArray(kURL, 50);
Serial.println(kURL);

/* @方法2 */
char kPath[128];

kPath[0] = 0;
strcat(kPath, "/some_static_code_here/plus?");
strcat(kPath, kAnotherParameter);

Serial.println(kPath);