browse-url-interactive-arg is a compiled Lisp function in `browse-url.el'.

(browse-url-interactive-arg PROMPT)

Read a URL from the minibuffer, prompting with PROMPT.
If `transient-mark-mode' is non-nil and the mark is active,
it defaults to the current region, else to the URL at or before
point. If invoked with a mouse button, it moves point to the
position clicked before acting.

This function returns a list (URL NEW-WINDOW-FLAG)
for use in `interactive'.