Powershell Add System Variable

Run PowerShell as an admin. Don't use this if you are trying to modify something like environmental extensions or environmental paths. No need to run refreshEnv and no need to open a new PowerShell window to see it

$variableNameToAdd = "mytestVariableName"
$variableValueToAdd = "some environmental value to add"
[System.Environment]::SetEnvironmentVariable($variableNameToAdd, $variableValueToAdd, [System.EnvironmentVariableTarget]::Machine)
[System.Environment]::SetEnvironmentVariable($variableNameToAdd, $variableValueToAdd, [System.EnvironmentVariableTarget]::Process)
[System.Environment]::SetEnvironmentVariable($variableNameToAdd, $variableValueToAdd, [System.EnvironmentVariableTarget]::User)

You can find a cool explanation of this below,

https://trevorsullivan.net/2016/07/25/powershell-environment-variables/

Run PowerShell as an administrator (to get the necessary registry access permissions) then call out to the .Net framework to set it:

[Environment]::SetEnvironmentVariable("MyTestVariable", "MyTestValue", "Machine")

NB. it won't take effect within the same process, you'll have to make a new PowerShell process to see it.