Scratch

Get the copy the url of the project. Do this by either going to the project, highlighting the url, right clicking and pressing "copy" or by going to a page with a link to the project (like your Stuff page) and right clicking on either the link or the thumbnail picture and clicking "Copy Link Location"

To put the link in a comment, simply paste it by right clicking and pressing "paste."

To put it in a forum, you also paste it, but then you have multiple options.
To make it look like this: http://scratch.mit.edu/projects/NESWario/1102405
do this:

[url]YOURurl.com[/url]

TO make it have words instead, like this, do this:

[url=YOURurl.com]Your text[/url]