XML file submission via API

Question

Good afternoon guys, I'm sending a POST request to an endpoint and in it I should send an xml file, but I'm returning a strange error, following the code is like this:

Public Function enviaxmlporto(ByVal diretorioXML As String, ByRef antt As String) As Boolean
    Try
        Dim client = New RestClient("*****")
        client.Timeout = -1
        Dim request = New RestRequest(Method.POST)
        request.AddHeader("Cookie", "portal[ses]=ab7478aed1848f085c6b3f61830258cc")
        request.AddHeader("Content-Type", "multipart/form-data")
        request.AddParameter("comp", "5")
        request.AddParameter("mod", "Upload")
        request.AddParameter("path", "eguarda/php/")
        request.AddFile("file","/C:/XMLaverbeporto/35220157460644000180550010008195051001305730.XML");
        Dim response As IRestResponse = client.Execute(request)
        Console.WriteLine(response.Content)
    Catch ex As Exception

    End Try
End Function

when I make the request it is returning the following exception

The given path format is not supported.

Can anyone tell me why this error?