Group based Authentication through Active Directory - restrict access to secured page

The code below works with my AD account groups and the login page goes to the page i need (mysecurepage.aspx) after I login using the username and password from the AD group.

However, im having a bit of trouble securing the page in preventing the user from being able to access the page through its URL outside of the login. Can't find an answer for this can anybody assist?

 Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    End Sub
    Public Class AllowedADGroup
        Public GroupName As String
        Public AssociatedUserType As String
    End Class

    Public Function UserHasPreferencesAccess(ByVal UserName As String, ByVal Password As String) As Boolean 
        Dim result As Boolean = False
        Dim referringPageName As String = "mystring"
        Dim actualADGroups As List(Of String) = GetGroups(UserName, Password)

        Dim matchingAllowedADGroup As AllowedADGroup = ADgroupsContainAnAllowedGroup(referringPageName, actualADGroups)

        If matchingAllowedADGroup IsNot Nothing Then
            'has a valid group
            result = True
        End If
        Return result
    End Function
    Public Function GetGroups(UserName, password) As List(Of String)
        Dim domainName As String = IPGlobalProperties.GetIPGlobalProperties().DomainName
        Dim context As PrincipalContext = New PrincipalContext(ContextType.Domain, domainName) 
        Dim usr As UserPrincipal = UserPrincipal.FindByIdentity(context, UserName)

        Dim result As New List(Of String)

        'verify credentials first
        If context.ValidateCredentials(UserName, password) = True Then

            ' if found - grab its groups
            If Not usr Is Nothing Then
                Dim groups As PrincipalSearchResult(Of Principal)
              
                groups = usr.GetAuthorizationGroups()
             
                ' iterate over all groups
                For Each p As Principal In groups
                    ' make sure to add only group principals
                    If TypeOf (p) Is GroupPrincipal Then
                        result.Add((CType(p, GroupPrincipal).Name))
                    End If
                Next
            End If
        End If

        Return result

    End Function

    Private Function GetAllowedActiveDirectoryGroups(ByVal requestingProductName As String) As List(Of AllowedADGroup)
        Dim retval As New List(Of AllowedADGroup)

        Dim sqlCon As New SqlConnection(ConfigurationManager.ConnectionStrings("ConString").ConnectionString)
        Dim sqlCmd As New SqlCommand("Client.GetAllowedActiveDirectoryGroups", sqlCon)

        sqlCmd.CommandType = CommandType.StoredProcedure
        sqlCmd.Parameters.Add("@RequestingProductName", SqlDbType.VarChar, 50)
        sqlCmd.Parameters("@RequestingProductName").Value = requestingProductName

        sqlCon.Open()
        Dim sqlr As SqlDataReader = sqlCmd.ExecuteReader()

        While sqlr.Read()
            Dim newADGroup As New AllowedADGroup

            If Not sqlr.IsDBNull(0) Then newADGroup.GroupName = sqlr.GetString(0)
            If Not sqlr.IsDBNull(1) Then newADGroup.AssociatedUserType = sqlr.GetString(1)

            retval.Add(newADGroup)
        End While

        Return retval
    End Function


    Private Function ADgroupsContainAnAllowedGroup(ByVal nameOfCallingProduct As String, ByVal adGroups As List(Of String)) As AllowedADGroup
        Dim retVal As AllowedADGroup = Nothing

        Dim allowedADGroups As List(Of AllowedADGroup) = GetAllowedActiveDirectoryGroups(nameOfCallingProduct)

        'Check user's AD security groups include one that's allowed e.g. "myPreferenceUsers"
        For Each grp As String In adGroups
            For Each allowedGroup In allowedADGroups
                If String.Compare(grp, allowedGroup.GroupName, True) = 0 Then
                    retVal = allowedGroup
                    Exit For 'Exit inner for
                End If
            Next
            If Not retVal Is Nothing Then
                Exit For
            End If
        Next

        If retVal Is Nothing Then
           
        End If

        Return retVal
    End Function

    Protected Sub btnLogin_Click(sender As Object, e As EventArgs)
              If UserHasPreferencesAccess(txtUserName.Text, txtPassword.Text) Then
            Response.Redirect("~/mysecurepage.aspx")
        Else
            messagebox.Text = "Login failed. Please check your user name and password and try again."


        End If

    End Sub