This feature can be enabled by users with Manager-level access or above.
Click Location Settings > Video Settings > Sales / Service / etc (choose the department you want to add the review button to):
Then, scroll down to the Thanks For Your Business video type (the Leave a Review button can only be added to this type).
In the 'Leave A Review Button' field, enter the URL you want the button to send the user to:
You can enter any URL for the button depending on where you want the customer to leave their review.
Here's how to find the URL for leaving a review on your Google Business Page:
You can also link to DealerRater, Facebook, Yelp, or anywhere else you want a customer to leave a review.