EchoSign does not support digital signatures. What do you mean by "add the signature request to a PDF". If you want to digitally sign PDF then how do you do it depends on PDF. If a PDF has unsigned signature fields, you click on the field and "Sign" dialog pops up and guides you through the process. If there are no unsigned signature fields in PDF then you select Fill&Sign->Work with Certificates->Sign with Certificate. You get a dialog to draw a rectangle for your signature field and then the same "Sign" dialog pops up. You need to have a certificate with a private key suitable for signing to digitally sign a PDF.