The only problem being that Reddit's OAuth requires the parameter duration to be set to permanent as a part of the authentication request in order to receive a refresh_token. duration seems to be a non-standard parameter as QOAuth2AuthorizationCodeFlow does not have a direct way to set the duration.

I tried using setModifyParametersFunction like so but it did not work

QOAuth2AuthorizationCodeFlow 

auto replyHandler = new QOAuthHttpServerReplyHandler(QHostAddress::Any, 1337, this);
oauth2.setReplyHandler(replyHandler);
oauth2.setAuthorizationUrl(QUrl(authorizationUrl));
oauth2.setTokenUrl(QUrl(accessTokenUrl));
oauth2.setClientIdentifier(clientId);
const QSet<QByteArray> scope = {QByteArray("identity"), QByteArray("read")};
oauth2.setRequestedScopeTokens(scope);

// Tried to add it here
oauth2.setModifyParametersFunction([](QAbstractOAuth::Stage stage, QMultiMap<QString, QVariant> *parameters) {
    if (!parameters)
        return; // Abort if pointer is invalid

    if (stage == QAbstractOAuth::Stage::RequestingAccessToken) {
        parameters->insert("duration"_L1, "permanent"_L1);
    }
});

Can somebody explain why this did not work? Is this an issue at the reddit side or did I just use the function incorrectly?