HTTPS: How can I build the body for an HTTPS Post?
You can build the JSON format using the Merge Texts action this way.
This should work to send multiple variables too. For more context, check this conversation from the slack community.
We're working on a feature called String Templating, that will allow users to build a string from multiple variables, more intuitively.
You can follow up and vote for that feature here: https://bardeen.canny.io/features/p/string-templating