I've tried and created a webhook and it seems that anyone can access it as long as you know the URL. It is possible to put an authorization for it?
The webhook security is defined "inside rundeck" I mean, you can define a user to execute the job and probably you need to create an ACL for giving access to execute. The webhooks include in the URL all auth info to be executed, in that sense, you cannot add a extra auth layer to the webhook, the idea of webhooks is to be used with other solutions, for example, trigger a job from PagerDuty.
If you need to execute jobs with "dynamic authentication" the best approach is to execute from API using the user token as a variable.
UPDATE 26/05/2022:
Now you can use a security token on your Webhooks calls, that's only available for PagerDuty Process Automation On Prem (formerly "Rundeck Enterprise"), take a look.