In order to protect your FoundryVTT server, we provide a default password. The Server Password
is shown on your server's details page. Simply enter that as the admin password when prompted by FoundryVTT and you should be able to login. Once logged in, you are able to set the FoundryVTT password to whatever you desire.
You can always override the default password set to login so that you don't have to copy and paste the default password. This can be achieved by starting your server and going to the Configuration section of the Setup area.
Once you've entered a new password, select "Save Changes".
/data/Config
.