WP Hook is a custom form action that acts as a callback to an action hook of your choice. You can use the WP Hook action instead of writing a full custom function of your own.
Using the WP Hook Action
The action is found on the Emails and Action tab of the form builder. Developer Mode must be enabled. To add the action to any form, click Add New Action and then the WP Hook action from inside the Actions Window that opens.
The action will now appear in the list. Click WP Hook to open its settings window.
The Hook Tag setting is where you can specify what WordPress action hook (stock or custom) to call.
The callback will trigger on form submission only. The callback cannot update the submitted form data as this is a WordPress action and not a filter.
During submission the specified Hook Tag is passed a $form_data array variable. This array contains the form id, form fields, and form settings data as top level keys in the array.