I installed SSMS 18. I installed a new LocalDB instance by choosing it from the SQL Server Express 2017 installation as it is not yet included in the SSMS 18 installation.>
I can't see the images due my shop's policies, but I think that you can achieve your goal in 2 ways.
In SSMS right click the server name and select properties, then go to "database settings" there's a section named "Database default locations" modify to meet your requirements.
As a second option is to specify the datafiles explicity like:
CREATE DATABASE [test]
ON PRIMARY
( NAME = N'test', FILENAME = N'c:\mypath\test.mdf' , SIZE = 8192KB , FILEGROWTH = 65536KB )
LOG ON
( NAME = N'test_log', FILENAME = N'c:\mypath\test_log.ldf' , SIZE = 8192KB , FILEGROWTH = 65536KB )