For the link provided the form has disabled dropdown but it has the options element in it so you may set disabled as false and can do further processing.
(The dropdown are not waiting for ajax request to get filled)
Sub test_fill_form()
Dim url1 As String
Dim url2 As String
Dim url3 As String
url1 = "https://auto."
url2 = "ar"
url3 = "do.ch/selling/?step=1&uniqueid=8a7d2327-f426-4df7-8291-d6b55fc62e3c"
Dim ie As Object
Set ie = CreateObject("InternetExplorer.Application")
ie.Visible = True
'apiShowWindow ie.Hwnd, SW_MAXIMIZE
ie.navigate url1 & "ric" & url2 & url3
While ie.readyState <> 4: DoEvents: Wend
ie.document.all("Article_ArticleDetails_AutoRegistrationMonth").Click
ie.document.getElementById("Article_ArticleDetails_AutoRegistrationMonth").Value = "1"
ie.document.getElementById("Article_ArticleDetails_AutoRegistrationYear").Value = "2013"
ie.document.getElementById("Article_ArticleDetails_AutoMakeId").disabled = False
ie.document.getElementById("Article_ArticleDetails_AutoModelId").disabled = False
ie.document.getElementById("Article_ArticleDetails_AutoFuel").disabled = False
End Sub