I need to register a new smart tag (custom). The smart tag should fetch a custom field related to the user profile that has filled out the form. The form uses the user_id field, so by passing this value as a parameter in a get_user_meta function, I can retrieve the custom field I need. The issue is that I can't find any documentation on the website, neither filters nor action hooks, that would allow me to perform this operation. Is there a hook to register a custom smart tag? So that it passes the form object as an argument, which contains the data from which I can extract the user_id and then construct the output for my custom shortcode?