Issue using custom triggers

The all you need to know is in the docs https://docs.bracketspace.com/docs/registering-custom-triggers/

But how can I integrate html file and php file for the button click event?

Most likely you need an AJAX call.

Can you suggest any video tutorial for this?

Hi, unfortunately, we have no video but you can also read this article: https://www.smashingmagazine.com/2018/07/wordpress-notifications-made-easy/

Can you guide me through general steps for the following scenario:
I’m having real estate wordpress theme. In ‘single-listing’ i.e., a custom post type template for showing property details, when a user(buyer) clicks a button for application of property, notification should be sent to the user(landlord) who has registered that property.

Well, I cannot really write more than in the docs or article. We can write this for you though.