@Maximilian313 --- You would NOT be under a Creative Commons Attribution. That would allow commercialization of your work and the source material, which is not the case. You can't give permission for people to use EK's works.
You would be writing under the doctrine of Fair Use. While copyright laws vary between countries, WattPad defers to US copyright law. That makes
this the governing law. But before we get into an overly detailed discussion of copyright law, here's the short part:
Fanfiction is legal, and allowed so long as it's non-infringing, not claiming to be the creator of the source material, and not being sold. Note, David the Developer even chimed in above with that exact standard. Disney and Starwars have some curious things to say about Fanfiction but those aren't controlling case law so long as the aforementioned requirements are met.
So it's not a case of you having a copyright over EK material (you don't), so much as it's a fair use matter where you don't permission (again, so long the conditions you meet).
As for your own unique creative work, you don't need to have a formal copyright registration. All written works are protected as soon as they are written. Just image if people needed to file for a formal registration for every written work.
So best advice. Read WattPad's terms and conditions on submitting fanfiction. Those can be found
here. Notice, that WattPad respect the rights of copyright holders to issue take-down notices.
So, less stress about copyright, and more stress on actually writing.