I assume you are asking about the SEF biometrics appointment notification (invite)?
If so, Portal das Financas has nothing to do with it. It is the name of the Autoridade Tributaria’s portal (Tax Authority).
For ARI purposes you need access to SEF ARI portal (or you rely on your lawyer’s access to the said portal).
Full guidance here:
Now if you have your own access to the SEF ARI portal, then yes, you can go in and grab the biometric appointment by yourself as soon as you get SEF’s invite by email.
The only thing to be aware of is that SEF sometimes sends emails to the applicant but sometimes only to their lawyer. E.g. I never received my pre-approval email (well, nobody received it because I did not have a lawyer at that stage) nor my final approval (that one my lawyer received and forwarded).