add_rewrite_tag()

add_rewrite_tag() — Adds a new rewrite tag (like %postname%). The $query parameter is optional. If it is omitted you must ensure that you call this on, or before, the {

Inputs: $tag (string): Name of the new rewrite tag. $regex (string): Regular expression to substitute the tag for in rewrite rules. $query (string): Optional. String to append to the rewritten query. Must end in ‘=’. Default empty.