I found the solution to my question. AccessToken can be obtained by this->myObject->token() and the Header can be set using
m_networkRequest.setRawHeader("Authorization", "Bearer " + this->myObject->token().toUtf8());
So my code snippet becomes
// private
// QOAuth2AuthorizationCodeFlow * myObject
connect(this->myObject, &QOAuth2AuthorizationCodeFlow::granted, [=](){
qDebug() << __FUNCTION__ << __LINE__ << "Access Granted!";
QJsonObject obj;
obj.insert("param1", "someval");
QJsonDocument doc(obj);
QString strJson(doc.toJson(QJsonDocument::Compact));
QNetworkRequest m_networkRequest;
m_networkRequest.setUrl(QUrl("API_URL"));
m_networkRequest.setHeader(QNetworkRequest::ContentTypeHeader,"application/json");
m_networkRequest.setRawHeader("Authorization", "Bearer " + this->myObject->token().toUtf8());
auto m_networkReply = m_networkAccessManager->post(m_networkRequest, strJson.toUtf8());
connect(m_networkReply, &QNetworkReply::finished, [=](){
qDebug() << "REQUEST FINISHED. Error? " << (m_networkReply->error() != QNetworkReply::NoError);
qDebug() << m_networkReply->readAll();
});
});