Add outline to text in Preview

Unfortunately, I was unable to find a way to exactly imitate the desired behavior. You may have to simply create the outlined text in a different application and import it into Preview.

However, while it might not be exactly what you're looking for, you can create text with a background color. To do so, bring up the markup toolbar and create a new text box. Then, using the dropdown menu indicated in the below screenshot, select a background color.

The font, along with its color and size, can be modified by selecting the "A" icon to the right of the aforementioned menu. If you want a literal outline around the entire text box, that can be accomplished by selecting the dropdown to the left of the previously mentioned background color menu.