Created a "login" dialog form to grab the server name from the user. Now a valid connection must be made before the Main form loads.

AdvertsingProfitControl/FrmDatabaseSelector.cs

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+using System;
+using System.Data.SqlClient;
+using System.Windows.Forms;
+using AdvertsingProfitControl.Properties;
+
+namespace AdvertsingProfitControl
+{
+    public partial class FrmDatabaseSelector : Form
+    {
+        public bool _connectionSuccessful = false;
+
+        public FrmDatabaseSelector()
+        {
+            InitializeComponent();
+        }
+
+        private void button1_Click(object sender, EventArgs e)
+        {
+            var connectionString = "Data Source=" + serverNameTextBox.Text + ";" + "Initial Catalog=AdvertisingProfitControl;Integrated Security=True;MultipleActiveResultSets=True;App=EntityFramework";
+            try
+            {
+                //Test the connection string with a forced timeout of 2 seconds.
+                using (var conn = new SqlConnection(connectionString + "Connection Timeout=2"))
+                {                   
+                    conn.Open(); // throws if invalid
+                }
+                Settings.Default.ConnectionString = connectionString;
+                _connectionSuccessful = true;
+                this.Close();
+            }
+            catch (Exception ex)
+            {
+                label1.Text = ex.Message;
+            }
+        }
+    }
+}