…initiate a direct web service call…
Does “initiate a… call” mean that you want to run a program on your AS/400 that will connect to some remote web service to request some data? Or are you wanting to have programming on your AS/400 that can act as a web service? I’d guess the first, but we should be sure.
Either way, you will definitely be involving ILE programming in order to use the necessary APIs. You might only write ILE COBOL wrappers for the APIs and call those wrappers from COBOL/400, but I can’t see any sense in writing for COBOL/400 at all.