function here()
{
var result = confirm("YOU WANT");
if(result)
{
__doPostBack('UNIQUE NAME OF TEXT BOX', null);
}
else
{
return false;
}
}
<asp:TextBox ID="TextBox1" Onchange="return here()" runat="server"
AutoPostBack="True" ontextchanged="TextBox1_TextChanged"></asp:TextBox>