Include the imports also:

using System.Data.Sql;
using System.Data.SqlClient;

protected void Button1_Click(object sender, EventArgs e)
{
   string name = TxtName.Text; // This is your textbox value
   

   string connString = ConfigurationManager.ConnectionStrings["NORTHWNDConnectionString"].ConnectionString;
   SqlConnection conn = null;
   try
   {
          conn = new SqlConnection(connString);
          conn.Open();

          using(SqlCommand cmd = new SqlCommand())
          {
                 cmd.Conn = conn;
                 cmd.CommandType = CommandType.Text;
                 cmd.CommandText = "INSERT INTO dummyTable(name) Values (@var)";
                 cmd.Parameters.AddWithValue("@var", name);
                 int rowsAffected = cmd.ExecuteNonQuery();
                 if(rowsAffected ==1)
                 {
                        //Success notification
                 }
                 else
                 {
                        //Error notification
                 }
          }
   }
   catch(Exception ex)
   {
          //Error message
   }
   finally
   {
          if(conn!=null)
          {
                 //Close connection
          }
   }
}