As you set application/x-www-form-urlencoded as content type so data sent must be like this format.

String urlParameters  = "param1=data1&param2=data2&param3=data3";

Sending part now is quite straightforward.

byte[] postData = urlParameters.getBytes( StandardCharsets.UTF_8 );
int postDataLength = postData.length;
String request = "<Url here>";
URL url = new URL( request );
HttpURLConnection conn= (HttpURLConnection) url.openConnection();
conn.setDoOutput(true);
conn.setInstanceFollowRedirects(false);
conn.setRequestMethod("POST");
conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
conn.setRequestProperty("charset", "utf-8");
conn.setRequestProperty("Content-Length", Integer.toString(postDataLength ));
conn.setUseCaches(false);
try(DataOutputStream wr = new DataOutputStream(conn.getOutputStream())) {
wr.write( postData );
}

Or you can create a generic method to build key value pattern which is required for application/x-www-form-urlencoded.

private String getDataString(HashMap<String, String> params) throws UnsupportedEncodingException{
StringBuilder result = new StringBuilder();
boolean first = true;
for(Map.Entry<String, String> entry : params.entrySet()){
if (first)
first = false;
else
result.append("&");
result.append(URLEncoder.encode(entry.getKey(), "UTF-8"));
result.append("=");
result.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
}
return result.toString();
}

For HttpEntity, the below answer works

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);


MultiValueMap<String, String> map= new LinkedMultiValueMap<String, String>();
map.add("email", "first.last@example.com");


HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<MultiValueMap<String, String>>(map, headers);


ResponseEntity<String> response = restTemplate.postForEntity( url, request , String.class );

For reference: How to POST form data with Spring RestTemplate?

string urlParameters = "param1=value1&param2=value2";
string _endPointName = "your url post api";


var httpWebRequest = (HttpWebRequest)WebRequest.Create(_endPointName);


httpWebRequest.ContentType = "application/x-www-form-urlencoded";
httpWebRequest.Method = "POST";
httpWebRequest.Headers["ContentType"] = "application/x-www-form-urlencoded";


System.Net.ServicePointManager.ServerCertificateValidationCallback +=
(se, cert, chain, sslerror) =>
{
return true;
};




using (var streamWriter = new StreamWriter(httpWebRequest.GetRequestStream()))
{
streamWriter.Write(urlParameters);
streamWriter.Flush();
streamWriter.Close();
}
var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();


using (var streamReader = new StreamReader(httpResponse.GetResponseStream()))
{
var result = streamReader.ReadToEnd();
}

Building off of Navoneel's answer, I like to use StreamEx's EntryStream.

Here is Naveoneel's method re-written.

private String getDataString(HashMap<String, String> params)
{
return EntryStream.
of(data).
mapKeys(key -> URLEncoder.encode(key, StandardCharsets.UTF_8)).        // Encode the keys
mapValues(value -> URLEncoder.encode(value, StandardCharsets.UTF_8)).  // Encode the values
join("=").                                                             // Create a key=value
joining("&");                                                          // Assemble into key1=value1&key2=value2...
}

Use Java 11 HttpClient

The HTTP Client was added in Java 11. It can be used to request HTTP resources over the network. It supports HTTP/1.1 and HTTP/2, both synchronous and asynchronous programming models, handles request and response bodies as reactive-streams, and follows the familiar builder pattern.

https://openjdk.java.net/groups/net/httpclient/intro.html

HttpClient client = HttpClient.newHttpClient();;
HttpRequest request = HttpRequest.newBuilder()
.uri(new URI(targetUrl))
.POST(urlParameters)
.headers("Content-Type", "application/x-www-form-urlencoded")
.build();
HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());

Problem with the Response!

I had a similar issue when I was sending the request of media type application/x-www-form-urlencoded from Postman I was receiving a correct response.

However, when sending the request using code, I was receiving jargon somewhat like:

�-�YO�`:ur���g�
.n��l���u)�i�h3J%Gl�?����k

What I tried:

Out of frustration for multiple days tried all the possible solutions from changing character sets to changing header values to code changes and whatnot.

Solution lies in Postman.

Select Java-OkHttp

Copy the code and paste it into IDE.

That's it.

Reference for HttpOk: https://www.vogella.com/tutorials/JavaLibrary-OkHttp/article.html