Instead, I would simply recommend that you use the label on your reference selector to guide your user.
For people who are searching for a solution for this, there is a custom widget in the appstore for this:
The widget adds a placeholder functionality as well as the creation of a stylable version of a dropdown selector and it's elements. This stylable version uses Bootstrap compatible markup and styling and is easily adjusted to the desired result. It is meant as an extension of the native Mendix dropdown selector elements such as reference selectors. Meaning you can still use all the normal, native options of the Mendix widget.