If you use the Xfce desktop, the Switch User functionality of its Action Button panel item specifically looks for the gdmflexiserver executable in order to enable itself. If you provide it with an executable shell script /usr/bin/gdmflexiserver consisting of
#!/bin/sh
/usr/bin/dm-tool switch-to-greeter
this is from https://wiki.archlinux.org/index.php/LightDM