My first SOAP client. I have constructed the XML message... how to send?

I'm using C# to write something to make it easier to post my company's jobs.

One of the job boards allows us to upload jobs using a webservice, and we have to send it a SOAP envelope.

(see section "Create the XML Request" on page http://partner.monster.com/real-time-posting-devguide )

I have constructed my XML as a System.String based on user input in a form. I based it on the XML example given by the webservice provider.

I haven't used SOAP before so I thought I could just POST the XML to the URL using HttpWebRequest, as I did this with another more basic webservice (see earlier post: https://www.experts-exchange.com/questions/28708426/use-C-to-attach-to-a-URL-and-get-an-XML-feed.html). However I got a 400 error.

So I googled sending a SOAP message and I got this:
https://msdn.microsoft.com/en-us/library/aa529276.aspx?cs-save-lang=1&cs-lang=csharp#code-snippet-3
But it doesn't allow (or so it seems to me) to just append the *already formed* XML message and simply send it (or am I wrong).

(on top of that the SOAPClient class is in microsoft.web.services3.dll which aint on my machine)

So maybe looking for another way...

Any tips guys?
thanks