Event Grid sys topic subscription

I'm trying to create with az cli an Event grid system topic subscription but it fails with the foloowing error:

"statusMessage": "{\"status\":\"Failed\",\"error\":{\"code\":\"ResourceOperationFailure\",
\"message\":\"The resource operation completed with terminal provisioning state 'Failed'.\",\"details\":[{\"code\":\"Url validation\",
\"message\":\"Webhook validation handshake failed for https://mylogicapp.azurewebsites.net/runtime/webhooks/workflow/scaleUnits/prod-00/workflows/84aa5895b4324710853436be49fed029/triggers/When_a_resource_event_occurs/paths/invoke.
Http POST request failed with response code Unknown. For troublehooting, visit https://aka.ms/esvalidation. Activity id:9d3516d8-75ee-4923-bf54-a19f15631fbf, timestamp: 1/16/2022 8:56:35 PM (UTC).\"}]}}",

`Connect-AzAccount
$subscription = 'subid'
$ResourceGroupName = 'myrg'
Set-AzContext -Subscription $subscription

az eventgrid system-topic event-subscription create --name 'testsub' `
-g $ResourceGroupName --system-topic-name my-systopic `
--included-event-types Microsoft.Storage.BlobCreated `
--subject-begins-with '/blobServices/default/containers/mycontainer/blobs' `
--enable-advanced-filtering-on-arrays true `
--endpoint "https://mylogicapp.azurewebsites.net/runtime/webhooks/workflow/scaleUnits/prod-00/workflows/84aa5895b4324710853436be49fed029/triggers/When_a_resource_event_occurs/paths/invoke?api-version=2018-11-01&code=myWorkflowcode&sp=%2Ftriggers%2FWhen_a_resource_event_occurs%2Frun&sv=1.0&sig=mysigvalue"`

but when I create the subscription manualy via Azure portal it works fine. I suspects the validation handshake via powershell is not taken care of url queries and stops at "Invoke"

For minformation my workflow test has a logic to validate the handshake to retun validation code.

any help please?