If you have a lot of files you need to add a prefix to there's a program called Bulk Rename Utility it's free and it does what the title says.
gci -filter *mp4 | ?{$_.name -notlike "^prefix"} | rename-item -newname {"prefix"+ $_.name} -whatif
Don't worry 007, we will get to it, you have my word.
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.management/get-childitem?view=powershell-7.3
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/where-object?view=powershell-7.3
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_comparison_operators?view=powershell-7.3
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.management/rename-item?view=powershell-7.3
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_pipelines?view=powershell-7.3
https://learn.microsoft.com/en-us/powershell/scripting/learn/ps101/04-pipelines?view=powershell-7.3
gci -filter *mp4 | ?{$_.name -notlike "prefix*"} | rename-item -newname {"prefix"+ $_.name} -whatif
This one gives a better result for me
Code:gci -filter *mp4 | ?{$_.name -notlike "prefix*"} | rename-item -newname {"prefix"+ $_.name} -whatif