https://ea.forgerock.com/docs/idm/objects-guide/policies-over-REST.html shows examples for validateProperty when the request is made directly to a specific user. There is special content required though when making a policy validation by REST to a resource url with "*".
The content must contain the object details (since they wont be read) as well as the properties which are being evaluated.
Custom conditional policy:
The request to validate policy with _action=validateProperty to a "*" resource path should look like the following (both object and properties may contain additional fields, this is just a minimal example that satisfied the above conditional policy):