CURL *curl;
CURLcode res;
curl = curl_easy_init();
if(curl) {
  curl_easy_setopt(curl, CURLOPT_CUSTOMREQUEST, "POST");
  curl_easy_setopt(curl, CURLOPT_URL, "https://api.swep.net/calculateSinglePhase");
  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, "X-API-KEY: YOUR_API_KEY");
  headers = curl_slist_append(headers, "Content-Type: application/json");
  curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
  const char *data = "{\r\n  \"inputs\": {\r\n    \"calculationMethod\": \"Design\",\r\n    \"selectedFluidSide1\": {\r\n      \"fluidId\": 53,\r\n      \"isGas\": false,\r\n      \"inputValue\": null\r\n    },\r\n    \"selectedFluidSide2\": {\r\n      \"fluidId\": 53,\r\n      \"isGas\": false,\r\n      \"inputValue\": null\r\n    },\r\n    \"coCurrent\": false,\r\n    \"selectedHeatExchangers\": [\r\n      {\r\n        \"heatExchangerInputType\": \"Parent\",\r\n        \"id\": 1\r\n      }\r\n    ],\r\n    \"heatLoad\": {\r\n      \"amount\": 1500,\r\n      \"unit\": \"HeatLoadUnit.Kilowatt\"\r\n    },\r\n    \"inletTemperatureSide1\": {\r\n      \"amount\": 80,\r\n      \"unit\": \"TemperatureUnit.DegreeCelsius\"\r\n    },\r\n    \"inletTemperatureSide2\": {\r\n      \"amount\": 20,\r\n      \"unit\": \"TemperatureUnit.DegreeCelsius\"\r\n    },\r\n    \"outletTemperatureSide1\": {\r\n      \"amount\": 40,\r\n      \"unit\": \"TemperatureUnit.DegreeCelsius\"\r\n    },\r\n    \"outletTemperatureSide2\": {\r\n      \"amount\": 50,\r\n      \"unit\": \"TemperatureUnit.DegreeCelsius\"\r\n    },\r\n    \"flowSide1\": null,\r\n    \"flowSide2\": null,\r\n    \"maxPressureDropSide1\": {\r\n      \"amount\": 20000,\r\n      \"unit\": \"PressureUnit.Pascal\"\r\n    },\r\n    \"maxPressureDropSide2\": {\r\n      \"amount\": 20000,\r\n      \"unit\": \"PressureUnit.Pascal\"\r\n    },\r\n    \"numberOfPasses\": null,\r\n    \"numberOfPlates\": null,\r\n    \"numberOfPlatesStack1\": null,\r\n    \"numberOfPlatesStack2\": null,\r\n    \"oversurface\": null,\r\n    \"portSwitch\": true,\r\n    \"autoperformance\": false\r\n  }\r\n}";
  curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);
  res = curl_easy_perform(curl);
  curl_slist_free_all(headers);
}
curl_easy_cleanup(curl);