Adding Custom Fields to Merge Tags

I have a custom post type “Newsletter” which has a custom field for article_type. The goal is to have this field in the json payload for a webhook call for Trigger “On Newsletter Publish” and “On Newsletter Saved as Draft” (for testing). In reading https://docs.bracketspace.com/notification/developer/triggers/adding-merge-tags-to-existing-triggers it is not clear where this code goes. Is there a complete step-by-step example outlining the full set of code required, in which php files, which functions, etc?

Thanks for your assistance.

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