Anonymous send via Sign API

We currerntly issue and send PDFs programmatically - all sent automatically from one email account.

If we integrate to Sign it seems we have to send them with a user's email address  - but we want to continue to send from one standard email address.

We need to do this with the Sign API, without any requirement to log in to the Sign GUI.

For each 'send' we do know who the user is, so we could push the user name through.

Can anyone suggest where we find tips/tutorials/pointers for how to do all this?

It's not clear what section to look under

Thanks