Hi Jack,
I understand your issue, but I am not sure I can give you a quick answer here based on my limited experience.
Your MB flow is something like the following:
MQInput Node --- > HTTP Request --- > MQOutput node
To solve your problem, I suggest you first build the following flow
HTTPInput Node -X- > HTTP Request --- > HTTPReply node
This flow basically add MB between your
SOAP client and Web Service. You can use your SOAP client to connect to this flow. After this flow is completed, set a break point in the "X" location and take a look at the message using debugger in WMB toolkit. Then you can see what headers are required to feed in the HTTP request node.
After that, you may need to have a JavaCompute node or ESQL node to create the necessary message that feed the HTTP request node.
Good luck,