There is no way to explicitly create backlinks. backlinks are computed from the links you create (or are created) inside each Hypermedia node.
When you create a reference inside "Document B" to "Document A" (a reference can be a link or an embed), Your local now add this link to a local index of all the cross-references between all the documents you have in your node.
This makes it possible then to ask for all the "Citations" for a particular document (Citations is the name we use to describe the documents that include a reference to another document).