How can I POST data to a url using QNetworkAccessManager

Since some parameters and values might need to be utf-8 and percent encoded (spaces, &, =, special chars…), you should rather use QUrl (for Qt 4) or QUrlQuery (for Qt 5) to build the posted string.

Example code for Qt 4:

QUrl postData;
postData.addQueryItem("param1", "string");
postData.addQueryItem("param2", "string");
...
QNetworkRequest request(serviceUrl);    
request.setHeader(QNetworkRequest::ContentTypeHeader, 
    "application/x-www-form-urlencoded");
networkManager->post(request, postData.encodedQuery());

and for Qt 5:

QUrlQuery postData;
postData.addQueryItem("param1", "string");
postData.addQueryItem("param2", "string");
...
QNetworkRequest request(serviceUrl);
request.setHeader(QNetworkRequest::ContentTypeHeader, 
    "application/x-www-form-urlencoded");
networkManager->post(request, postData.toString(QUrl::FullyEncoded).toUtf8());

Starting with Qt 4.8 you can also use QHttpMultiPart if you need to upload files.