Some systems require a simple header field without an value. 

e.g. UPnP Devices following curl request works:

curl  'http://HOSTIP:49000/igdupnp/control/WANCommonIFC1' -H 'Content-Type: text/xml; charset="utf-8"' -H 'Soachemas-upnp-org:service:WANCommonInterfaceConfig:1#GetAddonInfos' -d '@XMLBODY'