SqlCommand putuser = new SqlCommand("INSERT INTO MainDB(NumberofEmployees) VALUES(4)", cn);

putuser.ExecuteNonQuery();

<add name="ConnectionString"
     connectionString="Data Source=127.0.0.1; Initial Catalog=DataBase;Persist Security Info=True;User ID=user; Password=password

<add name="ConnectionString"
     connectionString="Data Source=127.0.0.1; Initial Catalog=DataBase;Trusted_Connection=Yes;Persist Security Info=True;"/>