@PostMapping(value="/createAccount")

public String createAccount(String user, String pass){

   var hashmap = readUserMapFromDisk(MYUSERS.PATH_TO_TXT_FILE);

   hashmap.putIfAbsent(user, pass);

   writeToDisk(hashmap, MYUSERS.PATH_TO_TXT_FILE);

   return "All done " + LocalDateTime.now();

}

EDIT: Just imagine the string args are in a more complex object that can be serialized to JSON