I am using a webservice from the sandbox environment. To use it, it is necessary to have certificates anabled. I added them in the project settings (tab: certificates), but these settings are only valid for running your local host. For running in the cloud these certificates should be added to the configuration file. The configuration file can only be maintained for cloud nodes and not for a sandbox From Mendix support I got the reaction that certificates could not be used in sandbox environment, but that it might be possible to interract with the Java trust store from custom java actions. is there someone who is using this solution with java trust store for the described problem.
Have a look at this thread, in the answer is explained how to load your certificates from a pfx file.
Be aware that you'll need to run the webservice from java as well.
Hope this helps you in finding a solution.