Connecting Adobe PDF Services API to Dropbox content

I am trying to implement an Azure Logic App that will combine files into a single PDF using Adobe PDF Services API.

Supposedly, the API can connect directly to documents in my Dropbox. 

External Storage for Adobe PDF Services APIs
Adobe PDF Services APIs have added support for accessing files from the external  storage of the client directly. The client can invoke the PDF Services API with Signed URLs and files will be automatically accessed from their storage.

A Signed URL temporarily provides access to a resource. Signed URLs contain user/authentication information, allowing users without credentials to access the given resource.

PDF Services APIs supports the signed URLs of input and output files in the request body. Please refer to the External Section in Request Body of each API for more details.

Dropbox

PDF Services APIs can connect with Dropbox using Signed URIs for downloading and uploading the files from Dropbox location without the need for any credentials or access token.

Supported URL Prefixes:

• https://<item-id>.dl.dropboxusercontent.com
• https://content.dropboxapi.com/

Please refer here for more details.

I am trying to use the first one: https://<item-id>.dl.dropboxusercontent.com and I assume that since they are referring to this as a prefix, there must be addition code that follows but the link they provide for more details takes me to Dropbox API documentation.

They also say "Please refer to the External Section in Request Body of each API for more details." but I don't find an External Section when reviewing the Adobe or Dropbox documentation. 

I would really like my HTTP request to Adobe to take the file directly from Dropbox without having to deal with getting the content and sending it to Adobe from within my Logic App. 

Then I am hoping to save the result right back to dropbox so I don't have to get file content and then write it to Dropbox inside my App. 

I am going to cross post this on the Adobe Forum as well.