In the course description field in course settings, you can click on the "link" button in the toolbar to add an external URL link. This will open the link in the existing window the user is in which can make navigation back to the LMS difficult.
You can edit the course description in HTML and include coding to open an external URL in a new tab to ensure the user does not lose access to their LMS portal. To do this, click the "SOURCE" button in the toolbar of the course description field, you will see the any existing text translate to HTML.
Use the following coding in your description:
<a href="URL HERE" target="_blank">YOUR TEXT HERE</a>
This will allow for the link to be opened in a new tab instead of the existing window. If populated correctly, the user will see the URL as linked text.