There is two ways to add a link to your hub.
1. Using Link capsule.
2. Use the text capsule and write the link you want in the capsule and click the link chain on the right. Its the one connected.
You can also write it all out in html too. Click html which is on the left in the write capsule. A new window will show up and there you can write the html. Don't forget to update it when you are done though.