string connectionString = מחרוזת ההתחברות שיש לך ב-MainDB.cs
 string queryString = 
         "Insert INTO t_name (col1, col2, col3)
 VALUES('" + textBox1.Text +"','"+ textBox2.Text +"','"+ textBox3.Text +"',')"
    using (SqlConnection connection = new SqlConnection(
               connectionString))
    {
        SqlCommand command = new SqlCommand(
            queryString, connection);
        connection.Open();
        command.ExecuteNonQuery();
    }