1 answer
Julie, you will have to use an HTML or Custom email template, not a Text template.
See here:
https://success.salesforce.com/answers?id=90630000000grHjAAI
Edit: Using the HTML one will be easier since you can use the hyperlink button. The Custom one will require you to make the hyperlink yourself in HTML code. The one in the link is an HTML template. (I know that's a bit counterintuitive.)