Links can be copied from anywhere on the web and pasted into a post.
For example, from a topic list, highlight the title of the topic you are interested in and copy it by pressing ctrl+c in your keyboard. Paste it in the topic composer, and you should get something like:
When this happens the link will be automatically converted to markdown.
[This is the topic title?](https://<your-topic-url>)
This requires that Admins have not disabled the
enable rich text pastesite setting.