The Adapter Settings are also not taken into account if I do not pre-populate the properties of the %Net.HTTPRequest object in my method and call the adapter method without it:

set status = ..Adapter.SendFormDataURL(..Adapter.URL,httpresponse,"POST",,"",pRequest.FHIRResourceAsJSON) 

Always port 443 is used.

Thank you Jeffrey! The hint concerning the DTL Test facility was indeed helpful because I have tried a lot of possibilties but I got not the expected result in the DTL Test facility.