The HelloSign API has a few options for you depending on your use case and if you would like to use templates or not. For instance:
When your site user needs to send a signature request based on a template, you will need to create an embedded signature request draft. This draft can be opened in an embedded iFrame to allow the user to preview the request prior to sending. Creating the signature request draft for previewing is also a perfect opportunity to populate any pre-defined merge fields that might have been added by the user during the template creation process.
The embedded iFrame will be displayed and the user will be given an opportunity to preview and make edits to the signature request prior to sending. Any custom_fields parameters supplied to the draft creation API request will be added to the document as well and these fields can also be edited and moved by the user. The user can then send the signature request by clicking "Continue" (or "Send" if the subject and message for the request have already been set).
If the user that is reviewing also needs to sign the request, they can make sure they are a signer listed to complete the request.
https://app.hellosign.com/api/embeddedTemplatesWalkthrough#EmbeddedTemplatesPreviewingASignatureRequest