Hidden Field with Default:post_meta

I am attempting to create a hidden field using the special hidden input field. I have created a form and added [hidden example_field default:post_meta], along with making a custom field called example_field on the page in question. No matter what I put in the meta value, I cannot get it to populate the hidden field’s value. Am I missing something? The documentation page seems to suggest this can be done (with Getting Default Values from the Context). I am using the latest version of CF7, and can make it at least give me a fallback default when the post_meta value doesn’t work, but I need to differentiate between 2 values depending on the page. Thanks for any help in advance!