Hi @r.cauquil,
Welcome to the Auth0 Community!
I understand that your Post-User Registration Action did not trigger after you created a user using the Management API.
After testing this myself, I did not find the same observations. Instead, my Post-User-Registration Action is working regardless if I created the user using the Auth0 Dashboard, Management API, or registering on the sign-up page.
In this case, I believe that you may have an issue with your Action/Flow. Could you try adding a console.log() statement at the top of your function and verify that your action is working in the Real-time Webtask Logs Extension?
And, could you please double-check that you added your Custom Action into your Flow and saved those changes?
Looking forward to your response.
Thank you.