If you leave off the scheme, such as [//forums.nesdev.com/viewtopic.php?p=187593#p187593 rainwarrior's question], then it'll behave as an HTTP link if the wiki was visited through HTTP or an HTTPS link if the wiki was visited through HTTPS. Wikipedia's help pages call this a "protocol-relative URL".
One can link in this way from a wiki page to a forum post but unfortunately not vice versa. I just tested, and the [url] and [url=] tags in the version of phpBB in use here do not recognize protocol-relative URLs.
Wiki - Test roms page
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