isql
SQL>CREATE DATABASE 'D:\data\test.fdb' page_size 8192↵
CON>user 'SYSDBA' password 'masterkey';↵

linux, macos
SQL>CREATE DATABASE 'server:\data\test.fdb' page_size 8192↵
CON>user 'SYSDBA' password 'masterkey';↵

-- use localhost on local machine