Connecting C# to SQL Server

Question

I am trying to connect a program to the database hosted on SQL Server, but am having problems..

When the program runs, the login screen is the first to appear, and from it I need to open the second screen with options to add payments and etc in the database..

My database has 3 "roles" where each one has permissions to change payment, change contract, etc.. They are: Owner, Manager and Associate, and 3 users with the proper passwords to which I have already given the proper "roles".. They are: mrlee(Owner), mngr(Manager) and asso(Associate).

How can I connect to the database through these users I created in SQL Server and "keep" that connection when I open the other program window which is where I will make the changes? I need to keep the connection so that each user only does what is allowed in their "role".

It’s my first time trying to connect a program to a database and I’m having a hard time..

The Database name is ML074539

The login window looks like this:

public MainWindow()
    {
        InitializeComponent();
    }

    private void loginBtn_Click(object sender, RoutedEventArgs e)
    {
        if (usernameBox.Text == "" || passwordBox.Password == "")
        {
            MessageBox.Show("Please, check if you've entered your username and password.");
        } else
        {
            using (SqlConnection dataConnection = new SqlConnection())
            {
                try
                {
                    string userName = usernameBox.Text;
                    string password = passwordBox.Password;

                    SqlConnectionStringBuilder builder = new SqlConnectionStringBuilder();
                    builder.DataSource = ".\\MSSQLSERVER";
                    builder.InitialCatalog = "ML074539";
                    builder.UserID = userName;
                    builder.Password = password;
                    dataConnection.ConnectionString = builder.ConnectionString;
                    dataConnection.Open();


                    MultiLeaseWindow mw = new MultiLeaseWindow();
                    mw.Show();
                }
                catch (Exception ex)
                {
                    MessageBox.Show("Unexpected error:" + ex.Message);
                }
            }
        }

    }

Multileasewindow is where is the form to add payment and etc..

When I press the "Login" button, regardless of the User Name and Password I use (as long as it doesn’t leave blank), I get an error saying that the server was not found or not accessible and etc..

I’ve done a lot of research, but I still haven’t found an answer I can make work.