Modify existing PDF and add Adobe Sign Signature Fields for signing

Looking at various solution online, it appears that you can use Widgets to get back a hosting html page for your website (embedded).  However, in my case, I have a pdf that was generated by one system, then I have a c# (can be any language) service which needs to edit the pdf, insert the signature fields in the appropriate locations, and then send it to Adobe Sign for the routing/signing process.

What is the recommended approach for programmatically edit a PDF to insert the required form fields for Adobe Sign to work and place the signatures in the right locations based on the routes?

Thanks