If you paste a link it will usually create a preview. You can link text by highlighting the text and clicking the editor icon that looks like a chain link.
Links can also be manually formatted like this, if you’re familiar with markdown:
[link text](https://artofmemory.com/)
There’s more information here: How to format text in the forum
You don’t need to remember all that information though — the simplest way to create a link is to just paste the URL into the post editor. 