Attach Proposal Document to eSignature attachment field without using a Print Action

The need is to be able to attach the generated proposal document to a eSIgnature file attachment attribute on a click of a button (Other than Print - A pop up box is not desired). Tried to achieve this by calling a SOAP API within a modify action using BML but there is no enough documentation for this use case. The challenges are:

1. Identify/access the generated proposal

1. Attach this to eSignature attribute using an API

Any insights/comments are highly appreciated!!

Thanks!