RegisterCompleter.ps1

# define an argument completer
$completer = {
  param($commandName, $parameterName, $wordToComplete, $commandAst, $fakeBoundParameter)

  if($null -eq $script:_listOfArgCompletions){
    # download list and populate $script:_listOfArgComplettions
    $script:_listOfArgCompletions = @('Apples','Oranges','Nectarines')
  }

  $script:_listOfArgCompletions -like "$wordToComplete*" |ForEach-Object {
    [System.Management.Automation.CompletionResult]::new($_)
  }
}

# register it
Register-ArgumentCompleter -CommandName Get-CustomThing -ParameterName Name -ScriptBlock $completer