powershell-validate-script.ps1

function test {
    [cmdletbinding()]
    param(
        [parameter(mandatory)]
        [validatescript({test-computer $_})]
        [string] $computername
    )

    write-host $computername

}

function test-computer {
    [cmdletbinding()]
    param(
        [parameter(mandatory,valuefrompipeline,position=0)]
        [string] $computername
    )

    $computers = get-adcomputer -filter * | Where-object name -eq $computername
    $computers.name -contains $computername
}

test -computername "WIN10"
test -computername "FIN10"