Go to your course. Click on the Search button and search for the assignment you would like to edit. Then, click on the assignment.
You will be redirected to the week where that assignment is located. Click on the Pencil to edit it.
Highlight the text where you want to add the link and click the Link icon.
Paste the URL in the blank space and select Open in new window. Afterward, click on Create link.
Scroll down and click Save and Display.