How to let access webhook on GitLab

It’ quite long time that I’m dealing with webhook.
I finally find the way to create it on GitLab
the application that need it is:
But when I try to match the request from to GitLab, I obtain an error. It seem like my webhook does not works.
Can I:

  • try (on GitLab) the correct work of it (I mean I would try the script I’ve made)
  • Do someone knows if I can run (or, better, I can ask an external service) it from outside GitLab.

please help me, my work is blocked