In RDP session, tray icon of Service-Mode VNC is hidden.
So connect to your VNC Server using default VNC password and then change your authentication to your desired password.
But, What’s my (username and) password?
VNC Server password-protects remote access out-of-the-box so your
computer is safe as soon as you power it on.
Each time you connect, you’ll be asked to enter the remote access
credentials for that computer:
If you have a Home subscription, enter the password you specified when
you installed VNC Server on that computer. If you have a Professional
or Enterprise subscription (or took a trial), enter the username and
password you normally use to log on to your user account on that
computer.