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.
Enter the following line:
Now save the file and reboot your pc. You will be able to log in as different user.
You can check lightdm documentation.