I’m not sure if Falkon browser supports the proprietary DRM widevine module by default.
That’s why such options are missing from the menu.
However, Falkon uses the chromium code, so perhaps it can be possible to run widevine.
I can not check it now, but I see reddit posts about the fact that widevine works. Here’s how to do: PSA: Falkon (QT5 successor to Qupzilla) supports widevine (netflix), html5 video, and pepperflash : linux