It is not possible to choose a default charset (defaults to NONE) when creating a database with the FBManager.
I propose a setter on it to define the default charset, something like the following
FBManager fbm = new FBManager();
Quoting the workaroung provided by Mark Rotteveel stack overflow (https://stackoverflow.com/questions/51715228/set-default-charset-on-a-database-created-with-fbmanager/51726114#51726114
As a workaround you will need to alter the default character set after creating the database.
For Firebird 3 and higher you can do this by using
ALTER DATABASE SET DEFAULT CHARACTER SET UTF8
See also Alter the Default Character Set in the Firebird 3 release notes.
For Firebird 2.5 and earlier you'll need to modify the system tables directly (this is no longer supported in Firebird 3):
UPDATE RDB$DATABASE SET RDB$CHARACTER_SET_NAME = 'UTF8'
Replace UTF8 with your intended default character set if it is a different character set.
Either statement must be executed as the database owner or as user SYSDBA.
Be aware, changing the default character set only has effect on columns created after the new default has been set. Existing (var)char columns will retain their old character set.