I recommend making the JSON object as a formula field . If you have a lot of text data, you may need to handle double quotes and some other characters with SearchAndReplace.
"{\"hello\": \"" & [field] & "\"} \n" &Escaping the formula is tedious, but in my opinion this will be easier than constructing it in Pipelines. Once the JSON is in place it would be easy to send each individual record via webhook.
If you really need to send a single array with all of the object records, this becomes a bit more complicated. I don't think Pipelines is equipped to easily create new JSON objects, but I don't use Pipelines that much so I could be wrong.
Let me know if sending each record individually is off the table.