C# and VB.NET

dotnet add package PWFAuth

<appSettings>
  <add key="PWFAuthAppSecret" value="YOUR_64_CHARACTER_APP_SECRET" />
</appSettings>

Imports PWFAuth

Public Class FormLogin
    Public Shared Client As PwfClient

    Private Async Sub btnSignIn_Click(sender As Object, e As EventArgs) Handles btnSignIn.Click
        Client = New PwfClient(ConfigurationManager.AppSettings("PWFAuthAppSecret"))
        AddHandler Client.SessionEnded, Sub(s, args)
                                            ' already on the UI thread
                                            MessageBox.Show(args.Message, "Signed out")
                                            Application.Exit()
                                        End Sub

        btnSignIn.Enabled = False
        Dim login = Await Client.LoginAsync(txtKey.Text.Trim())
        btnSignIn.Enabled = True

        If Not login.Success Then
            lblStatus.Text = login.Message       ' written for the customer
            Return
        End If

        Client.StartHeartbeat()                  ' the kill switch: never skip it
        FormMain.Show()
        Hide()
    End Sub
End Class

using PWFAuth;

var client = new PwfClient(secret);
client.SessionEnded += (s, e) =>
{
    MessageBox.Show(e.Message, "Signed out");
    Application.Exit();
};

var login = await client.LoginAsync(txtKey.Text.Trim());
if (!login.Success) { lblStatus.Text = login.Message; return; }

client.StartHeartbeat();

Dim user As JsonElement
If login.TryGetProperty("user", user) Then
    Dim exp As JsonElement
    If user.TryGetProperty("expires_at", exp) AndAlso exp.ValueKind = JsonValueKind.String Then
        lblLicence.Text = "Expires " & DateTime.Parse(exp.GetString()).ToLocalTime().ToString("g")
    Else
        lblLicence.Text = "Lifetime licence"     ' expires_at is null for lifetime keys
    End If
End If

If Not login.Success AndAlso (login.ErrorCode = PwfErrorCodes.HwidMismatch OrElse
                              login.ErrorCode = PwfErrorCodes.DeviceLimit) Then
    If MessageBox.Show(login.Message & vbCrLf & "Move your licence to this PC?", "PWF Auth",
                       MessageBoxButtons.YesNo) = DialogResult.Yes Then
        Dim reset = Await Client.ResetHardwareIdAsync(key, "Moved to a new PC")
        MessageBox.Show(reset.Message)
        If reset.Success Then login = Await Client.LoginAsync(key)
    End If
End If

Dim login = Await Client.AccountLoginAsync(username, password)
If Not login.Success Then
    MessageBox.Show(login.Message)          ' INVALID_CREDENTIALS, BANNED, PAUSED, EXPIRED, HWID_MISMATCH …
    Return
End If
Client.StartHeartbeat()

Dim r = Await Client.RegisterAccountAsync(username, password, email)   ' REGISTRATION_DISABLED when sign-up is off
If r.ErrorCode = PwfErrorCodes.KeyRequired Then                      ' "Sign-up needs a licence key" is on
    r = Await Client.RegisterAccountWithKeyAsync(username, password, key, email)
End If

Dim more = Await Client.RedeemKeyAsync(key)                          ' signed in: adds the key's time
Dim back = Await Client.RedeemKeyAsync(username, password, key)      ' signed out, works after expiry too
Dim c = Await Client.ChangeAccountPasswordAsync(username, currentPassword, newPassword)

Await Client.LogoutAsync()      ' ends the session; the key stays bound to this PC
Client.Dispose()