I'd recommend you look at some of the freely available backup code. Here are two possible options I would recommend, as both work on SQL Server Express Edition.
1) Ola Hallengren's backup code.
You can install Ola's code in a database on your server (ex, in a "DBA" utility database), and then backing up the databases will be done by executing a stored procedure. The stored procedure takes care of putting date/time stamps in the file name, and can also take care of cleaning up old backups (and more).
Then, you can use a PowerShell (or cmd file) to call Invoke-SqlCmd (or sqlcmd) to simply call the procedure with the proper parameters.
Invoke-SqlCmd -ServerInstance SERVER02 -Database DBA -Query "EXEC dbo.BackupDatabase @Databases='testdb'"
SqlCmd -S SERVER02 -Q "EXEC DBA.dbo.BackupDatabase @Databases='testdb'"
2) The Dbatools PowerShell Module
This module includes a simple Backup-DbaDatabase command that will perform the backup and include the date/time stamp in the file name. This has fewer bells & whistles than Ola's procedure, but if you don't need the bells & whistles, this simple solution might be just what you need.
Backup-DbaDatabase -SqlInstance SERVER02 -Databases testdb -Type Full -BackupDirectory \server2\backups