Another method, that worked for me was:
Click on the globe-key and choose the “terminal” mode of the keyboard (which is standard in the terminal app, but also useful to switch to the extended keyboard or to have Cut-Paste functionality in other apps) …
… there we have ctl and up-down keys available and the “>_” key right side of the space-barr, which we click next …
so see the function keys, too …
… and by clicking “Menu” key, we get the copy / paste / select Menu.
I did not install anything for to get that functionality (Librem 5 Evergreen Oktober 2021)