String query = "INSERT INTO dbo.SMS_PW (id,username,password,email) VALUES (@id,@username, @password, @email)";
using(SqlConnection connection = new SqlConnection(connectionString))
using(SqlCommand command = new SqlCommand(query, connection))
{
    //a shorter syntax to adding parameters
    command.Parameters.Add("@id", SqlDbType.NChar).Value = "abc";

    command.Parameters.Add("@username", SqlDbType.NChar).Value = "abc";

    //a longer syntax for adding parameters
    command.Parameters.Add("@password", SqlDbType.NChar).Value = "abc";

    command.Parameters.Add("@email", SqlDbType.NChar).Value = "abc";

    //make sure you open and close(after executing) the connection
    connection.Open();
    command.ExecuteNonQuery();
    connection.Close();
}