There is a guide here to disable auto-login after installation Lubuntu 19.04, but I couldn't find a way to enable it.
The only information about how to enable/disable it is as follows, but there is no details how to re-enable it.
If you have autologin enabled and want to disable autologin open the file /etc/sddm.conf as root. To do this run
and open /etc/sddm.conf. Then to disable autologin remove the entire line [autologin].
Existing /etc/sddm.conf config
user@linux:~$ cat -n /etc/sddm.conf 1 [Autologin] 2 Session=Lubuntu 3 user@linux:~$