I would like to become an Adobe Stock contributor. Before submitting my content (I am the author) I would like to know if it has to be registered first in a copyright office in order to show I own the copyright?
No. Registration just only proves that you are the author of the work in a more formal way. Copyright is attributed by doing a work, not by registering the work.
You just go ahead and upload your work. You may be required to provide property release for some of your works, but no formal copyright registration is required.
You may read-up on "Content Ownership" at Account and submission guidelines at Adobe Stock . The links will give you more details regarding what uploads will require Property, or Model Releases.
I hope your found this helpful