I succeed to create a shortcut to toggle the Favorites panel with the keymap file.
They say:
So the variable is "favsToggleVisibility".
The path of the keyboard-desktop.json of the profile is found there:
If there's no such a file we create it with "export"
I open it in a text editor (Kate for me)
and edited it.
The block to add is:
I choose [F3] like shortcut.
I import the keymap file in the profile.
The new line appears:
We have to restart Joplin one time or more.
Et voilà...