Activate variable inside user input [duplicate]

Task: make PowerShell 5 to read/activate user input that contains any variable. This doesn't work on both script defined and environment variables. E.g.:

$H=4
$testParA=Read-Host "Try to input enviroment variable `$Env:windir :" #user input is $H
$testParB=Read-Host "Try to input variable `$H :" #user input is $H

Write-Host $testParA
Write-Output $testParA

Write-Host $testParB
Write-Output $testParB

Result:

> $Env:windir
> $Env:windir
> $H
> $H

Expectation:

> C:\Windows
> C:\Windows
> 4
> 4

Original task: In a text generator PowerShell script, let users to customize text, and where to place the serial counter variable "$serial". E.g., "some text $serial some other text"

No-duplication: This question has been marked as duplicate from "https://stackoverflow.com/questions/1667023/expanding-variables-in-file-contents", but not really. The answer is surely duplicated, but the question part is very different (You have to know "expand variables" to be able to find this question from searching, which is why I was unable to get answers before posting this question), and could have other methods to achieve the same or a better result. In general, on closing this question will only cause more people who never heared of "expand variables" to further create similar questions. There is probably a better method than simply closing this question.