If using Desktop, click HERE.
If using Mobile Web (Not the App), click HERE.
To add a link to a post you're creating...
Log into your account and click the "Home Page" button.
Write your post in the "Post Something" box, and click "Add Link". Paste the URL link you want to attach in the box provided.
After pasting your link in the box, click "Attach", and then click "Share" to post.
Now that you know how to add a link to your post, head over to the site and give it a try by clicking HERE.