When you setup an API endpoint, you have the option to use raw body content for the body parameters

Raw body content refers to the ability for you to document APIs that have no named JSON parameters. Something like the following in cURL:

curl --request POST \
  --url https://httpbin.org/post \
  --header 'content-type: application/json' \
  --data '"string"'

This also works for other primitives, arrays and objects.