Try this
JavaScript
<script language='JavaScript' type="text/javascript">
function getwords() {
document.getElementById('TextBox2').value = document.getElementById('TextBox1').value;
}
</script>
Html (aspx)
<asp:TextBox ID="TextBox1" runat="server" ></asp:TextBox>
<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>
<asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
Code behind (aspx.cs)
protected void Page_Load(object sender, EventArgs e)
{
TextBox1.Attributes.Add("Onblur", "getwords()");
}
protected void Button1_Click(object sender, EventArgs e)
{
string test = TextBox2.Text;
}