If you are afraid of your password being stolen by a malicious proxy or by MySiga server, you can do a POST request to the end-point /login/raw where you will need to encrypt your password before sending it, as it adds a lot of complexity it will not be covered in this quick tutorial.
With your CPF and password ready, u can prepare and send the login request like so:
Now you're logged, so let's do a GET request to /academic/registration/browser end-point, this only requires the PHPSESSID cookie to be sent so it knows who u are, the PHPSESSID value is the same as the "client" field on the login response.
If you are using the browser or an API tester, the registration pdf will show up.
If you are not using a browser or an API tester, you may see the raw pdf data of the registration file, you can save this data to a file, set the extension of that file to .pdf, and then open it normally.
Extra step
If you are a person who loves to click on the logout button, u can also send a GET request to /login/logout destroy your session on the MySiga and on Siga 3. This also requires u to send the PHPSESSID, so MySiga knows which session to destroy. Do it like this: