Endpoint not found when testing the Personalized Docs Webhook

Hi, I am trying to set the Personalized Docs Webhook for my application.
I added enpoint and served it with ngrok. I tested that endpoint through the Postman and it's giving me correct result. It's working. Here is the CURL:
curl --location --request POST 'https://8407-85-114-46-238.eu.ngrok.io/webhook'
--header 'Content-Type: application/json'
--data-raw '{
"email": "test"
}'

But when I test it in the 3. step of Personalized Docs I am getting:
Endpoint not found!
Did you forget the path in your server URL (e.g., /webhook)?
With Network response:
400 status code and json response {type: "not-found"}

Can you provide me more information about why am I getting this response?