How to indicate, via API, where on a PDF customer should sign ?

You can either specify the location by x,y coordinates, use text anchor extraction, or use document extraction (extract pdf fields). How are you going to be using the API? Directly with it or with a SDK? Text Anchors: https://developer.esignlive.com/guides/feature-guides/text-anchors/ Document Extraction: https://developer.esignlive.com/guides/feature-guides/document-extraction/ - this one can be done with the naming convention recognized by eSignLive for full automation of the extraction or you can just use the position extraction portion of the feature and use your own field names - https://developer.esignlive.com/guides/feature-guides/position-extraction/ x,y coordinates: https://developer.esignlive.com/guides/quick-start/ - The Create and Send guides on this page will show how to position by specifying the size and location. Hope this helps.