c # – how to execute an action when a field is empty

I have two linked text boxes: subtotal and discount, so that if the subtotal is greater than or equal to 500, the reduction will be equal to 0.10.

private void txtsubtotal_TextChanged (object sender, EventArgs e)
{
if (Convert.ToInt32 (txtsubtotal.Text)> = 500)
{
txtdescuento.Text = "0,10";
}
if not
{
if (Convert.ToInt32 (txtsubtotal.Text) <= 499)
{
txtdescuento.Text = "0";
}
if not
{
if (txtsubtotal.Text == "")
{
txtdescuento.Text = "0";
}
}