OAuth code for backend Api in c#

You can sign up for a developer account. 

On the main account page, find Personal Preferences > Access Tokens

Then find the (+) button on the right above the table. 

Create a new one and use the Integration Key along with: "Bearer "

And use that as the Authorization Header. It should look something like:

Bearer 3AAA...........................

Test it on: REST API — Document e-signature Software — Adobe Sign

Using baseUris endpoint -> add the Bearer to the Authorization input. Then "Try it out!" button.