There is a nice howto in the mendix documentation that explains how to use webservices properly: https://world.mendix.com/display/howto25/Consume+a+web+service
Do not include the envelope and header in the custom header. Mendix takes care of that. A custom header can be.
<wsse:Security mustUnderstand="1" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" >
<wsse:UsernameToken>
<wsse:Username>{1}</wsse:Username>
<wsse:Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordText">{2}</wsse:Password>
</wsse:UsernameToken>
</wsse:Security>
You can use the button "Insert template" for that.