Hey Luke,
Adding custom fields is indeed technically possible, but the act of wiring up Event meta specifically to that plugin is something a bit outside of the scope of our support forums, as it sounds like you’d have to modify code within that Post Notifier plugin.
But in general, you can just use WordPress’ built-in meta functions to get event meta. Learn about the function get_post_meta() here, for example → https://developer.wordpress.org/reference/functions/get_post_meta/
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One other thing to note is that for some specific things, we also have “helper” functions that are even simpler than the WordPress meta functions.
For example, you mention getting the start date for an event – you can do that with a function called simple tribe_get_start_date()
Getting the Venue for a given event can be done as well by simply using tribe_get_venue()
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One last tip about all of this: to explore the available “helper functions” like tribe_get_start_date() and such, head into your The Events Calendar plugin files and navigate to this folder: src/functions/template-tags. Inside this folder are a bunch of named files, which indicate what type of helper functions are within them. So for example, the venue.php file here contains helper functions like the aforementioned tribe_get_venue() function.
I hope this all helps!
— George