In my Ubuntu 20.04 LTS, I installed the x11vnc server to access it from a remote computer. Along with the x11vnc server, I installed lightdm as a display manager instead of gdm3. Then I added my Ubuntu to a windows active directory. Everything went fine. But when I tried to log in as a different user, I found no field to put a new username and password.
If you face this situation, it can be solved easily by editing the lightdm config file. Edit the following file as the root user.
/etc/lightdm/lightdm.conf
Enter the following line:
[SeatDefaults] greeter-show-manual-login=true
Now save the file and reboot your pc. You will be able to log in as different user.
You can check lightdm documentation.