Do you have sample code for performing an XML post for the inflight payment process?

I'm trying to post an XML file using an html form for the inflight payment process but I get a "system unavailable" error.  Is there any sample code for performing an XML post?

@using (Html.BeginForm(null, null, FormMethod.Post, new { @action = Model.PostUrl }))
{
@Html.HiddenFor(m => m.XmlData)

<row>
<div style="text-align: center">
<button type="submit" class="btn btn-success" style="width: 80px;">Post</button>
</div>
</row>
}