Search REST API with the User Profile Service and Hyphenated Last Names

Using the Search REST API to get all users where their last name begins with a certain letter to make a custom user directory. For example, I hoped that the following would return all users that have a last name that starts with A:

/_api/search/query?querytext=%27lastname:A*%27&sourceid=%27B09A7990-05EA-4AF9-81EF-EDFAB16C4E31%27 

However, that A* is really looking for words that start with A, not last names that start with A, so the above will return users with hyphenated last names where the second part starts with A. So the above will return “Carter-Adams” as part of the result set, which is not what I want.

How do I search to get values where the entire value starts with a certain string, rather than where any word in the value starts with that string?