Alright, I see several issues here.
The Set-Cookie header is usually returned by a web server to a web browser, in order to instruct the latter to store the cookie. The web browser then sends the value previously returned in the Set-Cookie header in subsequent calls, using the Cookie header (not Set-Cookie.)
So, if your token API returns 3 Set-Cookie headers, you need to send three Cookie headers to API X.
From what you wrote, I tend to understand you’re trying to send these cookies to the API X in request body, which probably isn’t right.
If you’re making a call to an IS service in a custom extension, you should try to inject these headers directly in the IS service.
One more question: what do you pass to the token API request in order to get these cookie? Do you need to pass it some credentials coming from the API caller? Or do you pass it always the same credentials?
If your token API does not check credentials of the caller, but is just in charge of giving you credentials to call the backend, then you should probably place your custom extension in the request processing stage.
#API-Gateway#API-Management#webMethods