Piping into kubectl.exe

I am trying to run a command like this:

kubectl get namespaces -o custom-columns=:metadata.name --no-headers 
       | Where-Object{$_.StartsWith("test") -or $_.StartsWith("dev")} 
       | kubectl get namespaces

(NOTE: the line breaks are just for readability, there are no line breaks in my actual command.)

The end goal is to apply a label to a subset of namespaces, but for now, I am just trying to feed the filtered piped list of spaces back in to kubectl so it will get them.

But I can't seem to get kubectl to accept the piped list as the list of namespaces to return. (this just returns all the namespaces on my cluster).

How can I get kubectl to take the pipe as input? (So I can filter the list of namespaces.)

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The kubectl docs make no mention of pipeline (stdin) input, so it may not be supported.

You state that passing multiple names as arguments can be achieved with appending a space-separated list of names to kubectl get namespaces

Therefore, you can simply collect the names in an array - which PowerShell automatically does when you collect an external-program call's output in a variable, with each output line becoming its own array element^[1] - and use it as an argument:

$namespaces =
  kubectl get namespaces -o custom-columns=:metadata.name --no-headers | 
  Where-Object { $_.StartsWith("test") -or $_.StartsWith("dev") } 

kubectl get namespaces $namespaces

PowerShell automatically passes an array's elements as individual, (stringified) arguments to external programs.

^{[1] If there happens to be just one line of output, it is collected as-is, i.e., as a [string] rather than a single-element array containing that string. However, the approach still works in that case. If you explicitly do need an array, use something like [array] $namespaces = ...}