SharePoint Designer Get an Excel Value (REST)

I have the following problem: I want to get the values of excel cells with the REST Api and in SharePoint Designer 2013. So to do this I created a workflow with the “Call HTTP web service” action. But now I’m a little bit stuck somewhere:

1. The URL: I’ve found some REST endpoints for Excel here. But I cannot find the endpoint for cell values. :-/

2. I have problems with the request header. To get a better feeling for the Excel Api I just wanted to start by getting the titles of all workbook sheets (with the “/Model/Sheets?$format=atom” endpoint). For this I created the following RequestHeader Dictionary:

But when logging the ResponseCode to Workflow History List I only get “Forbidden”. I thought it was a similar problem like this so I added the “Authorization: (empty string)” to the Request Header, but it did not work. :-/

So why is my request forbidden? :-/

Maybe someone can help me with this two little questions o: