I think you might need to create objects within the API Body block rather than using just text. This topic has some examples:
Also here (search for registration method within the page to see the image I’m suggesting):
It’s possible that it will work with only text blocks but you’d first need to get it working as a browser url (that is, you’re able to retrieve the json text response in a browser such as Chrome without even using Thunkable). If you have a working url that includes all of that info, share it and we can help you build the blocks to create it in Thunkable. Does Draftbit provide the full POST url, something like you might see with curl?
All that being said, there are some problem with your syntax. APIs generally use = instead of : and you would not include the brackets { } or quotes " ".