Hi @Andrea,
You do not need to build a webhook to perform http POST authentication with a custom-function.
Basically any provider can be authenticated via the following url:
https://realm.mongodb.com/api/client/v2.0/app/<yourappid-abcde>/auth/providers/<provider type>/login
For custom function you can do:
curl --location --request POST 'https://realm.mongodb.com/api/client/v2.0/app/myapp-abcde/auth/providers/custom-function/login' \
--header 'Content-Type: application/json' \
--data-raw '{
"username" : "myuser@exmaple.com"
}'
This will result in a successful authentication and the access token to use for other services (eg. graphql).
Remember to change your appId and provide the relevant payload for the auth provider, in your case a username field.
Please let me know if you have any additional questions.
Best regards,
Pavel