To request a review you push the changes into a special branch namespace in the reviewer's repository. IntelliJ uses / as a way to organize branches into folders, so it's easy to see what review requests come from who. You can also inform them in other ways (CCing on a ticket, email, chat, etc).

We use a kernel model in which there is no shared repository, so it's impossible for branches to be merged without being reviewed because it's the reviewer who merges them when they're satisfied.