ExcelREST API Unauthorized in workflow only (browser commands work OK)

Question

I am attempting to pull a range of cells from an excel spreadsheet in a SP 2013 doc library.  The URI is as follows 
https://<site>/_vti_bin/ExcelRest.aspx/<dir/filename>/Model/Ranges('Sheet1!E1%7CE100')
The URI produces clean results when entered into a browser address bar,
including all variants (discovery, sheets, charts, formats).  But
when I use a SP 2013 workflow it always returns "Unauthorized" as the Response Code.
The workflow (calling HTTP GET) is in a list on the same site as the doc library, and other (HTTP POST) workflows from this same list have written plain text to the target doc library, so no auth issues in seemingly similar circumstances.
I tried creating a null "Authorization" string as part of the Request Headers dictionary variable, as advised by other experts on the web, however that has not made a change in the results.  Also using "application/json;odata=verbose"
for the "Content-Type" and "Accept" strings withing my Request Headers, but the presence or absence of those two strings has no effect either.
TIA for any advice.

Answer 1

Hi,
According to your post, my understanding is that you get “unauthorizaed” issue in the workflow.
To use the POST method, we should include the X-RequestDigest
and Content-Length headers in the workflow.
I recommend you can first use the Fiddler to compose the HTTP POST method with the properly request headers and request body, then create your workflow.
There are two articles about using the Post method, you can refer to them.
http://www.fabiangwilliams.com/2013/09/04/using-the-call-http-web-service-rest-calls-in-sharepoint-online-using-spd/
http://blog.appliedis.com/2014/10/09/sharepoint-designer-2013-workflow-working-with-web-services/
Thanks & Regards,
Jason
Jason Guo
TechNet Community Support