How to make a string in AppSettings and use it later

Only use ConfigurationManager.AppSettings[chave] .

using System.Configuration;

// outras coisas 

public static void AlgumaFuncao()
{
    var valor = ConfigurationManager.AppSettings[exemplo];        
    meuTextBox.Text = valor.ToString();
}

Remembering that the application config needs to look like this

<configuration>
  <appSettings>
    <add key="exemplo" value="Alguma string de exemplo"/>
  </appSettings>
</configuration>

In your app.config :

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <appSettings>
    <add key="exemplo" value="exemplo"/>
  </appSettings>
</configuration>

No C # code:

string exemplo = System.Configuration.ConfigurationManager.AppSettings["exemplo"];

Remember to add a reference to System.Configuration

To put in a txtbox you should do this:

txtbox.text = Properties.Settings.Default.teste;

To save it looks like this:

Properties.Settings.Default.teste = txtbox.Text;
Properties.Settings.Default.Save();
Properties.Settings.Default.Upgrade();