backup to NAS using sqlcmd
[soapbox]


  I would tell them "for security reasons" you should provide Windows
  Authentication access to that storage. Ask them if they really want
  authentication information in plain text saved to access a box that stores all the data for the organization and would give a user full access to their data (unless you are encrypting your backups)?


[/soapbox]

For your specific issue the parameters in sqlcmd for authentication are to access a SQL Server instance, not to access external storage or other resource. You will need to adjust the scripts to more or less map the drive with the required credentials and then use that path within your backup script.

If you are on SQL Server 2014 I would suggest using PowerShell script or SQL Agent step as this contains easier methods of working with the file system and mapping drives. You can quickly check access to the path before running the T-SQL code to ensure it does not fail prematurely.