Insert from URL don't allow adding header, so it is not useful in the case.
I wonder if you could use a php script on your own web server to take the data you POST to it from FileMaker, and then format and send it to the service you require using CuRL or something.... It could then return a result that you could use in FileMaker...?
Using SOAP (http://en.wikipedia.org/wiki/SOAP), usually means a two-way transmission. A 'request' is sent and a 'response' returned (in a specific XML format/grammar). You cannot do this "natively" with FileMaker using any means. However there are plug-ins which can assist you with this transmission.
That, or as skywillmott states, you can use your own web site to create a "service" to do the transmitting (PHP or other web apps). You may or may not need ISP permission to do this.
adding 'content headers' as user19752 says, would be MOST helpful for HTTP POST (and with email sending). Send your requests in...
You can do SOAP easily when using CURL, e.g. via MBS Plugin.
We have examples for using web services included with plugin download.