Storing credentials of external services

I am developing an application that connects to multiple external services (mostly SOAP interfaces) that require authentication with username and password on the external side.
What are the best practices on the OutSystems platform regarding storing these usernames and passwords so they can be used in the actions calling the external services?
Can site properties be used for these fields? Is that secure?

Hi Roel,

Whether site properties are secure enough depends on how many people have access to the site properties, and whether the people that do have access can abuse the usernames/passwords and/or are likely to.