Set dynamically created Windows Form TextBox value on Button Click using C# and VB.Net

Refer below code.

Code

C#

protected void Button1_Click(object sender, EventArgs e)
{
    (((sender as Button).Parent as FlowLayoutPanel).Controls[1] as TextBox).Text = (sender as Button).Text;
}

private void OnAdd(object sender, EventArgs e)
{
    FlowLayoutPanel panel = new FlowLayoutPanel();
    Button btn = new Button();
    btn.BackColor = Color.DarkGray;
    btn.Height = 21;
    btn.Width = 100;
    btn.Text = textBox1.Text;
    btn.Click += new EventHandler(Button1_Click);
    panel.Controls.Add(btn);

    TextBox txt = new TextBox();
    txt.Name = "txt";
    txt.Height = 21;
    txt.Width = 100;
    panel.Controls.Add(txt);
    FlowLayoutPanel1.Controls.Add(panel);

    textBox1.Text = "";
}

VB.Net

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)
    CType(CType(CType(sender, Button).Parent, FlowLayoutPanel).Controls(1), TextBox).Text = CType(sender, Button).Text
End Sub

Private Sub OnAdd(sender As Object, e As EventArgs) Handles button1.Click
    Dim panel As FlowLayoutPanel = New FlowLayoutPanel
    Dim btn As Button = New Button()
    btn.BackColor = Color.DarkGray
    btn.Height = 21
    btn.Width = 100
    btn.Text = textBox1.Text
    AddHandler btn.Click, AddressOf Me.Button1_Click
    panel.Controls.Add(btn)

    Dim txt As TextBox = New TextBox()
    txt.Name = "txt"
    txt.Height = 21
    txt.Width = 100
    panel.Controls.Add(txt)
    FlowLayoutPanel1.Controls.Add(panel)

    textBox1.Text = ""
End Sub

Screenshot