If you are using a link that opens in a new window, it is a good idea to add a notification to the user that they will be leaving the current page and going to a new window in their browser.
Currently, the standard is to use this icon or one like it --> 
To add it to your links in your Guides, you can A. Add a link and choose the resource icon or B. Add the HTML of the icon to your source code.
This is the HTML you can use for that icon:
<i class="fa fa-external-link-square" aria-hidden="true"></i>
A. STEPS WHEN USING ADD LINK TO BOX:

- Add your link information
- Add Link Name
- Add Link URL
- Choose "Open in New Window"
- Choose the correct icon asset

B. STEPS WHEN USING ADD RICH TEXT/HTML:
- Add content with Rich Text/HMTL option or edit your current box to add a link

- Add the text that you would like to link and select the text

- With your text highlighted, click on the link
in the toolbar and add your text's URL/link information:

- Click on the "Target" tab and choose "New Window (_blank)". This is important so your link opens in a new window!

- Edit the Source to add the markup after your link

- Find your link and add the HTML for for the icon

Example:
Before:
Here is my link
After:
Here is my link