Figured I'd circle back to this in case anyone else had the same question. I booked an Economy flight through the portal. Just plain economy, not basic economy.

When trying to cancel on the United website, it says I must contact Capital One. On the Capital One website, it provides the option to cancel, but says I will receive no credit whatsoever. I used the chat on the United website and the agent said the third party allowed changes, so they asked what flight I wanted to change to. I replied that I just wanted to cancel and receive a voucher, which they did no problems.

I know everyone's experience has been a little bit different, so hopefully that helps someone.