Try this

<configuration>
 <connectionStrings>
  
   <add name="MyDbConn" 
        connectionString="Server=MyServer; Database=pubs; User Id=MyUser; password= P@ssw0rd"
        providerName="System.Data.SqlClient" />
 </connectionStrings>
</configuration>

Read the connection string from your code behind as below

string dbConn = ConfigurationManager.ConnectionString["MyDbConn"].ToString();

Add below code to execute the sql query

protected void Page_Load(object sender, EventArgs e)
  {
      using (SqlConnection cn = new SqlConnection(ConfigurationManager.ConnectionStrings["MyDbConn"].ToString()))
      {
          SqlCommand cmd = new SqlCommand("SELECT COUNT(*) FROM authors", cn);
          cn.Open();
          SqlDataReader rdr = cmd.ExecuteReader(CommandBehavior.CloseConnection);
          rdr.Read();
          Response.Write(rdr[0].ToString()); //read a value
      }
  }