How to make an Azure DevOps Pipeline wait for an external process to complete?

To make the Pipeline launch and then wait for my external process, I chose the path of least resistance and coded it as a PowerShell Task.

The external process is controlled through a REST API. Launching is done via a POST request and then a loop keeps polling the API with a GET request to see if the work is done. If a certain amount of time has passed without the process finishing successfully, the loop is aborted and the task is failed.

Here's the gist of my code:

$TimeoutAfter = New-TimeSpan -Minutes 5
$WaitBetweenPolling = New-TimeSpan -Seconds 10

# Launch external process
Invoke-RestMethod ...

$Timeout = (Get-Date).Add($TimeoutAfter)
do
{
    # Poll external process to see if it is done
    $Result = Invoke-RestMethod ...
    Start-Sleep -Seconds $WaitBetweenPolling.Seconds
}
while (($Result -eq "IN_PROGRESS") -and ((Get-Date) -lt $Timeout))

if ($Result -ne "SUCCESS")
{
    exit 1
}

PS - It's a good idea to sprinkle meaningful Write-Host messages in the above code, to make it easier to debug it when running in the pipeline.