API + set user to Preview Document

Hi, 

I am running an integration between servicenow and Adobe sign. I want to enforce the preview document so the manager can set digital signature as the only sign option! otherwise, it is not legal signed.

When I sent out a document for signing, the only person that is allowed to preview the document is the user/pass combination that we use in the authentication method.
with which api call can we change the "Sender" or can we authenticate another user the preview the document and set the digital signature field?

It MUST be with an api call!

thank you in advance,

Gert