How can I build the body for an HTTPS Post?

You can build the JSON format using the Merge Texts action this way.

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This should work to send multiple variables too. For more context, check this conversation from the slack community.

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We're working on a feature called String Templating, that will allow users to build a string from multiple variables, more intuitively.

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You can follow up and vote for that feature here: https://bardeen.canny.io/features/p/string-templating