How to populate textbox when listbox item is selected? C#

Question

Beginner to C#. I am trying to create a simple program that will populate text in a textbox based on the listbox entry that is selected once the button is pressed. I am trying different things but can't seem to find a way to get the text to populate in the text box. Code below. Thanks in advance.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();


        List<string> RadioList = new List<string>();
        RadioList.Add("Test1");
        RadioList.Add("Test2");
        RadioList.Add("Test3");


        listBox1.DataSource = RadioList;

    }

private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
private void RPODecoderButton_Click(object sender, EventArgs e)
    {

        if (listBox1.SelectedItem == "Test1")
        {
            RPOTextBox.Text = string.Empty;
            RPOTextBox.AppendText("Hello");
        }

        else if (listBox1.SelectedItem == "Test2")
        {
            RPOTextBox.Text = string.Empty;
            RPOTextBox.AppendText("Hi");
        }
}