When commenting, you copy the content of the address bar of your browser (that thing which starts with http://), and paste it in your comment. Link will be done automatically. But don't spam on other projects, it's not very polite. Use Show and Tell forums.
To post a link on forums, copy the content of the address bar of your browser and paste it like this:
[url]http://scratch.mit.edu/something[/url]
Which will produce http://scratch.mit.edu/something, or you can type:
[url=http://scratch.mit.edu/something]My project[/url]
Which will produce My project
Of course, you replace http://scratch.mit.edu/something with your projects web address (which will look like http://scratch.mit.edu/projects/filo5testing/794146).
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