How do I build connection with database....

{
                SqlConnection Con = new SqlConnection("Server=.;" + "database=Registration;"+"integrated Security=true;");
            }
            string Query = "Insert into[FormRegistration] values('1','dawood','Ahmed','Khan','Dubai','454','70','A')";
            SqlCommand sqlCommand = new SqlCommand(Query,Con);
            Con.Open();
            int result = sqlCommand.ExecuteNonQuery();
            Con.Close();
            MessageBox.Show($"{result} rows inserted");
}

private void btn_Click(object sender, EventArgs e)
        {
            string Error = "";
            if (string.IsNullOrEmpty(txtbox1.Text))
            {
                Error = "Please Enter Valid Id";
            }
            if (string.IsNullOrEmpty(txtbox2.Text))
            {
                Error = "Please Enter Valid First Name";
            }
            if (string.IsNullOrEmpty(txtbox3.Text))
            {
                Error = "Please Enter MiddleName";
            }
            if (string.IsNullOrEmpty(txtbox4.Text))
            {
                Error = "Please Enter  LastName";
            }
            if (string.IsNullOrEmpty(txtbox5.Text))
            {
                Error = "Please Enter Address";
            }
            if (string.IsNullOrEmpty(txtbox6.Text))
            {
                Error = "Please Enter Marks";
            }
            if (string.IsNullOrEmpty(txtbox7.Text))
            {
                Error = "Please Enter Percentage";
            }
            if (string.IsNullOrEmpty(txtbox8.Text))
            {
                Error = "Please Enter Grade";
            }
            if (Error.Length > 0)
            {
                MessageBox.Show(Error, "Validation Error", MessageBoxButtons.YesNo);
            }
            else
            {
                SqlConnection Con = new SqlConnection("Server=.;" + "database=Registration;"+"integrated Security=true;");
            }
            string Query = "Insert into[FormRegista] values('1','dawood','Ahmed','Khan','Dubai','454','70','A')";
            SqlCommand sqlCommand = new SqlCommand(Query,Con);
            Con.Open();
            int result = sqlCommand.ExecuteNonQuery();
            Con.Close();
            MessageBox.Show($"{result} rows inserted