How would you do HMAC request signing before triggering a webhook ?

One of my providers protects webhook calls using HMAC based authentication.

See here for details.

It requires to compute a signin string before issuing the request.

1. Can it be done ? (I need an external security library to compute the signin code)
2. Can we use a variable that isn't necessarily a user property ?

Note: this works great with Postman for instance.