Enable PowerShell to run command line commands

IT will depend on what commands you want to run.

it should be

& dir

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PowerShell does its best to interpret the command line, especially when variables are involved.
This works most of the times, but obviously has its limits.
For example, PS "removes" (it actually doesn't pass them along) double quotes it finds unnecessary, while adding them when it thinks they are (that is, whenever there's a space in the variable).
That's best shown with the good old "find.exe", because it insists on the search string being surrounded by quotes.
Very simple command, and not really what you'd expect as result:

PS C:\> "IT Guy" | find.exe /i "T"
FIND: Parameter format not correct
PS C:\> $t = 'T'
PS C:\> "IT" | find.exe /i $t
FIND: Parameter format not correct

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Here, PS decided that the double quotes were not required, so it removed them before passing the arguments to find.exe, and find.exe didn't like it at all.
So for comparison, let's see what happens when a space is involved:

PS C:\> "IT Guy" | find.exe /i "T G"
IT Guy
PS C:\> $tg = 'T G'
PS C:\> "IT Guy" | find.exe /i $tg
IT Guy

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Workaround 1: force the quotes, either by embedding the argument (including the double quotes) in single quotes, or by embedding escaped double quotes:

PS C:\> "IT Guy" | find.exe /i '"T"'
IT Guy
PS C:\> "IT Guy" | find.exe /i "`"T`""
IT Guy
PS C:\> $t = 'T'
PS C:\> "IT Guy" | find.exe /i "`"$t`""
IT Guy

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Workaround 2 (requires PS 3.0 or later): tell PowerShell to mind its own business and let you do the thinking, by telling it to stop parsing the command line at "--%". Everything following that string will be taken literally.
This is pretty much what you asked for initially.
Note that the "stop parsing" means exactly that: you can't use variables after that signal anymore, either:

PS C:\> "IT Guy" | find.exe /i --% "T"
IT Guy
PS C:\> $t = "T"
PS C:\> "IT Guy" | find.exe /i --% "'"$t`""
File not found - '$T`
PS C:\> "IT Guy" | find.exe /i --% "$t"
## No output, because now it's looking for the literal string "$t"
PS C:\> '$T' | find.exe /i --% "$t"
$T

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Now, did I just say you can't use variables anymore? That's not the whole truth. There's a workaround for that, too. After the "--%" (and only after that), PS will expand environment variables enclosed in percent signs, just like cmd.exe does. So set an environment variable to whatever you need, and it looks nearly like good old cmd.exe:

PS C:\> $env:T = "T"
PS C:\> "IT Guy" | find.exe --% /i "%T%"
IT Guy

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Finally, if the path to an external command requires quotes because it contains spaces, or if you want to define the path in a variable, you have to use the "&" operator to tell PS that you want to invoke the string, not just send it to the pipeline.
Personally, I usually use the "&" when calling external programs, whether required or not, to make it clear what's happening, and always add the extension as well.

PS C:\> $env:T = "T"
PS C:\> "IT Guy" | & "C:\Windows\system32\find.exe" --% /i "%T%"
IT Guy
PS C:\> $find = "C:\Windows\system32\find.exe"
PS C:\> "IT Guy" | & $find --% /i "%T%"
IT Guy

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See Get-Help about_Parsing as well
About Parsing
https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_parsing?view=powershell-5.1

If all that doesn't help, there's still Start-Process, where you can build your own command line arguments just like in VBScript.

Edit: added information about the "&" operator.