ValidateScript is very powerful to validate script / fucntion parameters, example below validate country based on rest service

ValidateCountry.ps1

Param(
    [ValidateScript(
    {
        $countries = ((Invoke-RestMethod -Uri https://raw.githubusercontent.com/devlead/ISO-3166-Countries-with-Regional-Codes/master/all/all.json)| % Name)
        if ( $countries -contains $_)
        {
            $true
        }
        else
        {
            throw "Valid countries:`r`n`t$($countries -join "`r`n`t")"
        }
    }
    )]
    [string]$Country
)
"You chosed an valid country : $Country"