URL above = the page that bootstraps the AJAX call (we extract the WP nonce from its inline JS).
How to fill this in: open the venue's events page, open DevTools → Network, click the "View More" / "Load More" button, find the admin-ajax.php POST. Copy its form-data fields and translate them like this:
action=load_grid_events&nonce=6eba3e53ec&search=&presenter=&grid=12&genre=&page=1
becomes:
{"action":"load_grid_events","extra_params":{"search":"","presenter":"","grid":"12","genre":""}} Rules: drop nonce (we extract it from the page) and page (we paginate). Everything else goes in extra_params, including empty strings — some plugins (showpro/Texas Performing Arts/Long Center) reject the request if filter fields are missing. Optional: add "nonce_var": "<js_object_name>" to constrain nonce extraction to a specific inline-JS object (e.g. "event_object"), or "ajax_path": "/wp-admin/admin-ajax.php" to override the default.