Inserting hyperlink fields into emails and tasks
When you take a sharepoint hyperlink field and put it into a task form or email, by default the value of the field is converted to string and looks like this:
Salesforce - from side bar
By default, it should take the display text of the hyperlink and build a link out of it with the URL
Josh Moore commented
Furthermore, there is no way to build a hyperlink from the hyperlink field using the hyperlink button in the designer, because the properties of the fields are not available in the insert hyperlink button.