Set password on SQL Server backup file

Use the backup database command with PASSWORD option. Sample is shown below;
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BACKUP DATABASE DBName TO DISK='c:\DBName.bak' WITH INIT,PASSWORD='abc'

Restore with password
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RESTORE DATABASE DBName
FROM DISK = 'C:\DBName.bak'
WITH PASSWORD = 'abc',
MOVE 'DBName_Data' TO 'd:\MSSQL\MSSQL\DATA\DBName_Data.MDF',
MOVE 'DBName_Log' TO 'd:\MSSQL\MSSQL\DATA\DBName_Log.LDF'