Newbie: Where to file (End to end encryption key) E2EE mnemonic in Linux desktop?

Howdy!

I set up my Nextcloud vps by web front end and iOS Nextcloud client. In the latter, after installing the end to end encryption app in the NC server, I set up several test folders locked to e2ee. I confirmed these worked in the web end.

I then spent a flustery time getting the current release of the Linux desktop client working on a Raspberry Pi 4B running Raspbian. After dependency yak shaving, I finally got it to compile from GitHub with submodules.

It works, save that I cannot find where to input my e2ee mnemonic key so that the NC desktop client will download and decrypt my test folders. Is there a config file? Searching online, I found that there is supposed to be an infobox on the settings page, but this does not display. Right clicking on the folders in the desktop client widget page reveals nothing. The arrow revealing only gives an error message for those folders which are encrypted. The desktop client sees the server, sees the files, but has nowhere to input the mnemonic key.

Whilst setting up, I got a pop up asking for a login name and password to create a keyring. Ought I have filed the mnemonic there? There was no info on where to file the mnemonic in that, nor what the pop up was actually for.

Any help with this is warmly welcome.