Sandbox's Webhooks works with webhookinbox but do not reach the server with ngrok

[philippe4doma]

Dear developer community, 
I am new to authorize.net and have an issue with webhooks.
it works fine with webhookinbox, but when I tried with ngrok (http tunnelling) my server did not receive anything.

{

    "status": 500,

    "reason": "PING_FAILED",

    "message": "An error occurred while processing the request. Please note the correlation id for the request and contact Support for more details.",

    "correlationId": "99097dd9-e402-4692-80a7-5b51ab0e3449",

    "details": [

        {

            "message": "Error occured in connecting to the endpoint: https://499f-2a01-e0a-2db-95b0-df3-64e0-6dab-7fba.eu.ngrok.io/webhook/authorize. "

        }

    ]

}

While it works fine when i call the same url directly with curl 

curl https://499f-2a01-e0a-2db-95b0-df3-64e0-6dab-7fba.eu.ngrok.io/webhook/authorize -X POST -H 'x-anet-signature: foo' -H 'expect: 100-continue' -d "bar"

I suspect it is due to the "expect: 100-continue" in the header, but I am not sure and I do not know how to disable this from authorize.net ?
Perhaps I am doing something wrong, but the support told me to reach the developer community.
Has anybody already use Authorize.net's webhooks with ngrok ?

If someone can help me, it would be great.

Thanks in advance