How to get the data from database using connection string

Imports System
Imports System.Collections.Generic
Imports System.Web
Imports System.Xml
Imports System.Windows.Forms
Imports System.Data.SqlClient
Imports System.Configuration


Public Class Form1

    Private Sub ComboBox1_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox1.SelectedIndexChanged
        Dim VB = ConfigurationManager.ConnectionString["HMDB"].ConnectionString
        Using Con = New SqlConnection(VB)
            Dim SqlText = "Select DocEntry from dbo.Master1"
            Dim cmd = New SqlCommand(SqlText, Con)
            Con.Open()
            ComboBox1.DataSource = cmd.ExecuteReader()
            ComboBox1.DataBind()

        End Using
    End Sub
End Class

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <connectionstrings>
  <connectionStrings>
      <add name ="HMDB"
           connectionString ="Data Source=DESKTOP-68VGRJG;Database=master;integrated security =SSPI"/>
            </connectionStrings>
</configuration>
</configuration>