Rather than registering a protocol, you may opt to publish it in a journal. Protocols for systematic reviews are widely accepted for publication; a limited number of journals publish protocols for scoping reviews.
The advice for publishing systematic reviews also applies to publishing protocols. A journal that accepts systematic reviews may also accept protocols. Perhaps the journal's editors can suggest where to register/publish a protocol before submitting the review for publication.
PROSPERO is an international registery of systematic review protocols. The journal JBI Evidence Synthesis accepts systematic reviews and protocols.
See the links below for more ideas about where to submit. In addition, some subject-specific journals may publish protocols. You will need to check the scope and author information for relevant journals.
You have a few options when publishing a review: