Can I use azure virtual machine as a webhook endpoint to receive event from event grid

I have built an webhook server on a azure VM by python flask.

But when I add this endpoint on storage account event grid, I got following error:

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Deployment has failed with the following error: {"code":"Url validation","message":"The attempt to validate the provided endpoint https://xxx.xxx.xxx.xxx/webhook failed. For more details, visit https://aka.ms/esvalidation."}

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As I check this flask web server log, there is no request sent to my server.

Then I use wireshark to capture network packets, there is only SSL client hello message, then the client close the connection without sending request.

My flask server use self-signed certificates, is the self-signed certificates not available as a webhook server for event grid?