Putting a # allows for named category linking, while topics become named when using the url, and replies include the post number after that same url.
Examples:
#project-skyscraper -> category link
named topic link: https://forums.atlas-65.com/t/wiki-post-possibly-useful/9407
named reply link: https://forums.atlas-65.com/t/wiki-post-possibly-useful/9407/25
Posting urls on their own on a **separate line** creates a 'Onebox' instead (mind the spacing).
https://forums.atlas-65.com/t/wiki-post-possibly-useful/9407/25
Result from above:
Project Skyscraper → category link
named topic link: Wiki Post - possibly useful?
named reply link: Wiki Post - possibly useful? - #25 by DevilinPixy
Posting urls (external too) on their own, on a separate line creates a ‘Onebox’ instead
Note: A Onebox is a more advanced preview of the content linked to. It could result in text, image, video, and more. The result will occupy more space, which can be useful in some cases. Be mindful of the result and judge for yourself.
Another way to create links, is to ‘mask’ a url. This can be very useful to shorten a very long url, or to allow for linking within a sentence, or otherwise create a descriptive alternatively named link. In general we do recommend to include the :link: 
emoji when a url is not local, but external, possibly including the site name within parenthesis after.
Examples:
[Expedition Countdown](https://jsfiddle.net/DevilinPixy/7jt945Lq/) :link: (JSFiddle)
See patch notes [here](https://forums.atlas-65.com/t/updates-patches/8822/111).
Result:
Expedition Countdown (JSFiddle)
See patch notes here.