How do I electronically sign a document my Firm sent me through the Portal?

Great question, first lets cover some basics:

"E-sign" actually means a physical signature on a signature-pad connected to a computer at the Firm's walk-up customer counter (or on a Firm owned tablet carried by an authorized employee). This is not what we are after in this article..

A simple 'DocuSign' MAY be embedded into the form (IF your Firm chose to do so).
IF a DocuSign capable form exists for you on the Portal, you can "sign" the form on the Portal from your device and save the changes.
BE AWARE: Even if it were available in the past, DocuSign may no longer be available from your Firm, as DocuSign is no longer sufficient for remote electronic signatures on tax documents according to https://www.irs.gov/pub/irs-pdf/p1345.pdf (see pages 21-24).
Similar government policy updates may affect other classification Firms which handle federally regulated documents.

There are 2 ways to send signatures (IRS compliant) through our Portal:
1. You print the form, physically sign it, and scan it back in to the portal (using a scanner or taking QUALITY pictures with your camera phone). You must retain the hardcopy for your records in the event of an IRS audit.
2. The Firm sends you a PDF (with identity challenge questions embedded, which you have to answer correctly)* that you will complete and save in the Portal.

*Your Firm's internally used softwares may not have the ability to produce a challenge-question-enabled PDF, or furthermore their softwares may actually prevent them from creating challenge-question-enabled PDFs via any means. This all has to do with liability and Federal compliance.

Consult with your Firm how they recommend you get a signed document returned to them.
The ability to sign a form online is a function of the form creation software, not of the Portal housing the form, and may be bound by governing regulations.