you need to place an empty invoke step in your flow and then fill in the service name to invoke by copy&paste or typing manually.
Check the output of the service to see how exactly the variables are structured and named.
As the service does not have an appropriate List ACL it will not be possible to call or select it directly in IS Admin UI and Designer, only indirectly by creating a flow service invoke step as described above.
Due to these restrictions you should use this service with caution as it might change in future releases without any notice.
This is true for all services starting with “wm." instead of "pub.”, even if they are visible in Designer or can be selected & tested in IS Admin UI.
When ever possible such services should not be used directly by any means, but you are encouraged to check if there is a way to do this with services being publicly available.