I would have thought that there was a "Form load" rule template available at the "content" level, but I can't see it. Could this perhaps be available at a different level? Or do I need to execute my rule as a "page change" rule?
Note that if you're using a template that does not allow access to the top-level element (as should usually be the case) then the template designer should have set this up as a form option, accessible through the Form Options dialog.
The form load rule is available when you select the top-level form element.