Box sign API : change/create sign box documents using prepare_url

Question for sign document preparation process :

1 : as example use case flow is req. to change/create sign documents using prepare_url. but it is only change by admin(service account/user) not any other user or a assign signer email address(signer 1, 2). can anyone provide information or process for prepare_url , is_document_preparation_needed options for sign request? like how it works with permission.

2. I need to change as recipient email(for first signer or any box user email) who is create/change sign request documents first then send it.  Example : "Recipient Email" need to change with signer email not box login account/service api email. currently auto set by fina_copy_reader using api.