Umlauts problem when sending Json Post with VS2017 C++ using libcurl - c++

Question

when posting data to zendesk, all of my umlauts are saved as "?" characters.
the same post from the postmann app works fine.
i even copied the whole postman C - libcurl code, compiled and fired it up:
same result, umlauts are shown as questionsmarks.
Whats the difference between postman posting the json and visual studio posting it?
here is the code:
CURL *curl;
CURLcode res;
curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
curl_easy_setopt(curl, CURLOPT_URL, "https://xx.zendesk.com/api/v2/users/create_or_update.json");
curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
curl_easy_setopt(curl, CURLOPT_DEFAULT_PROTOCOL, "https");
struct curl_slist *headers = NULL;
headers = curl_slist_append(headers, "id: 1");
headers = curl_slist_append(headers, "Authorization: Basic eXaMpLe1234");
headers = curl_slist_append(headers, "Content-Type: application/json");
headers = curl_slist_append(headers, "Cookie: __cfruid=2c9eb500bc16cc34cd7390604b24ece125e7e8ad-187187187");
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
const char *data = "{ \"user\": {\"external_id\": \"160102\", \"name\" : \"TestÜÄÖ\", \"phone\": \"014567-0\", \"user_fields\": { \"anrede\": \"Herr\"}, \"organization\":{\"name\":\"testorg\"}, \"email\" : \"\"} }";
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);
res = curl_easy_perform(curl);
}
curl_easy_cleanup(curl);
is there a configuration in VS that i have to do to tell him i want utf-8 encoding when posting data?
"charset utf-8" in header yields the same results
Edit: sending a basic curl with the same data using windows cmd behaves the same and saved umlauts as questionmarks, even tho postman generated that curl statement which works when postman sends it.

Answer 1

for people googling:
the u8 prefix as commented by Botje worked