Сменить UserAgent в элементе WebBrowser

 Declare Sub UrlMkSetSessionOption Lib "URLMON.dll" (
     ByVal dwOption As Int32,
     ByVal pBuffer As String,
     ByVal dwBufferLength As Int32,
     ByVal dwReserved As Int32)
 
    Const URLMON_OPTION_USERAGENT As Integer = &H10000001
    Const URLMON_OPTION_USERAGENT_REFRESH As Integer = &H10000002
 
       Dim ua As String = "Mozilla/5.0 (Linux; Android 7.1.1; Moto G (5S) Plus Build/NPSS26.116-45-5; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/63.0.3239.111 Mobile Safari/537.36 Instagram 27.0.0.11.97 Android (25/7.1.1; 480dpi; 1080x1776; motorola; Moto G (5S) Plus; sanders_nt; qcom; pt_BR)"
 
'Mozilla/5.0 (Linux; Android 6.0.1; ZUK Z2131 Build/MMB29M; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/48.0.2564.106 Mobile Safari/537.36 Instagram 62.0.0.19.93 Android (23/6.0.1; 480dpi; 1080x1920; ZUK; ZUK Z2131; z2_plus; qcom; pt_BR; 123790722)
'Mozilla/5.0 (iPhone; CPU iPhone OS 11_4 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Mobile/15F79 Instagram 52.0.0.14.164 (iPhone8,2; iOS 11_4; pt_BR; pt-BR; scale=2.61; gamut=normal; 1080x1920)
'Mozilla/5.0 (Linux; Android 6.0.1; SM-J500M Build/MMB29M; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/55.0.2883.91 Mobile Safari/537.36 Instagram 40.0.0.14.95 Android (23/6.0.1; 320dpi; 720x1280; samsung; SM-J500M; j5lte; qcom; pt_BR; 102221278)
 
        UrlMkSetSessionOption(URLMON_OPTION_USERAGENT_REFRESH, Nothing, 0, 0)
        UrlMkSetSessionOption(URLMON_OPTION_USERAGENT, ua, ua.Length, 0)
 
        MsgBox("done")

WebBrowser1.Navigate("http://сайт", Nothing, Nothing, "User-Agent: Mozilla/5.0" & vbCr & vbLf)

Imports System.Runtime.InteropServices
Public Class Form1
    <DllImport("urlmon.dll", CharSet:=CharSet.Ansi)>
    Private Shared Function UrlMkSetSessionOption(ByVal dwOption As Integer, ByVal pBuffer As String, ByVal dwBufferLength As Integer, ByVal dwReserved As Integer) As Integer
    End Function
 
    Const URLMON_OPTION_USERAGENT As Integer = &H10000001
 
    Public Function ChangeUserAgent(ByVal Agent As String)
        UrlMkSetSessionOption(URLMON_OPTION_USERAGENT, Agent, Agent.Length, 0)
    End Function
 
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        ChangeUserAgent("Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A537a Safari/419.3")
        WebBrowser1.Navigate("https://www.instagram.com/", "_self", Nothing, "User-Agent: Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A537a Safari/419.3")
    End Sub
End Class

Public Class ExtendedWebBrowser
        Inherits WebBrowser
 
        Private renavigating As Boolean = False
        Public Property UserAgent() As String
 
        Public Sub New()
            AddHandler Me.DocumentCompleted, AddressOf SetupBrowser
 
            'this will cause SetupBrowser to run (we need a document object)
            Navigate("about:blank")
        End Sub
 
        Private Sub SetupBrowser(ByVal sender As Object, ByVal e As WebBrowserDocumentCompletedEventArgs)
            RemoveHandler Me.DocumentCompleted, AddressOf SetupBrowser
            Dim xBrowser As SHDocVw.WebBrowser = CType(ActiveXInstance, SHDocVw.WebBrowser)
            AddHandler xBrowser.BeforeNavigate2, AddressOf BeforeNavigate
            AddHandler Me.DocumentCompleted, AddressOf PageLoaded
        End Sub
 
        Private Sub PageLoaded(ByVal sender As Object, ByVal e As WebBrowserDocumentCompletedEventArgs)
 
        End Sub
 
        Private Sub BeforeNavigate(ByVal pDisp As Object, ByRef url As Object, ByRef flags As Object, ByRef targetFrameName As Object, ByRef postData As Object, ByRef headers As Object, ByRef cancel As Boolean)
            If Not String.IsNullOrEmpty(UserAgent) Then
                If Not renavigating Then
                    headers += String.Format("User-Agent: {0}" & vbCrLf, UserAgent)
                    renavigating = True
                    cancel = True
                    Navigate(DirectCast(url, String), DirectCast(targetFrameName, String), DirectCast(postData, Byte()), DirectCast(headers, String))
                Else
                    renavigating = False
                End If
            End If
        End Sub
    End Class