bookmark-completing-read is a compiled Lisp function in `bookmark.el'.

(bookmark-completing-read PROMPT &optional DEFAULT)

Prompting with PROMPT, read a bookmark name in completion.
PROMPT will get a ": " stuck on the end no matter what, so you
probably don't want to include one yourself.
Optional arg DEFAULT is a string to return if the user input is empty.
If DEFAULT is nil then return empty string for empty input.