Single square brackets. The first space is interpreted as the end of the URL and anything after that shows up as text. You should look at Help:Links.
The syntax is usually like this:
[sitename.com This is a site] gives...
First, add a single square bracket, then copy the URL and paste it after the bracket. Next, if you want different link text, add a space, then add link text. Finally, the external link ends with a single closing square bracket.
If you want to add a link to another wiki, the syntax is usually like this:
[[w:c:dev:RoundAvatars]] gives...
First, add "[[w:c:" (without quotation marks), then add the wiki's URL name (e.g. the wiki URL name for the FANDOM Open Source Library is "dev") then it's done. If you want to link to a specific page in a different wiki, add a colon (:), then the exact name of the page. Finally, you end the link with 2 closing square brackets.
External links also work with different communities, but the interwiki links are a quicker and more professional way to link to them.
See Help:Links for full information.