1. As advised by good people, I create a file in the user's home folder # vi /usr/home/clios/.xinitrc of such content:
#!/bin/sh
exec startxfce4
exec fluxbox
exec wmaker
2. Installing the graphical login manager # pkg install x11/sddm
3. Edit the file # vi /etc/ttys, the 8th terminal
ttyv8 "/usr/local/bin/sddm -nodaemon" cons25r on secure
4. # kill -HUP 1
4.1 # echo 'sddm_enable="YES"' >> /etc/rc.conf
delete the slim line in /etc/rc.conf
5. # service sddm restart
6. When starting sddm, enter the user password: XXXXXXXXXX
Special thanks to the masters: Alexander8820, eternal_noob