<configuration>
    <appSettings>
        <add key="url" value="domain.de" />
        <add key="user" value="name" />
        <add key="password" value="geheim" />
    </appSettings>
</configuration>

        TextBox1.Text = _
          System.Configuration.ConfigurationSettings.AppSettings.Get("url")
        TextBox2.Text = _
        System.Configuration.ConfigurationSettings.AppSettings.Get("user")
        TextBox3.Text = _
        System.Configuration.ConfigurationSettings.AppSettings.Get("password")