I got it to work. I first created the subscription in vRA that had the criteria I wanted.
Then I created a test workflow that had a scriptable task in it. The input parameter is a vCACCAFE:WorkflowSubscription. The code in the scriptable was simple:
var criteria = vCACCAFEEBSHelper.getCriteriaFromWorkflowSubscription(workflowSubscription);
Running this logged out the criteria I needed as a string.
To make my life simple in the workflow running the "Register a tenant workflow subscription", I created another scriptable to populate the criteria:
var criteriaObj =
// pasted output from running the test workflow mentioned above
var criteria = JSON.stringify(criteriaObj);
As for the version, I left it null. The priority I set to 'PUBLISHED'
I updated to vRA 7.0.1 and started getting an error about the version. Instead of null, I know have to set it to "0.0.0".