This post shows you How to make a placeholder for a 'select' box.
For example:
<select>
<option value="1">C#</option>
<option value="2">C++</option>
</select>
If you want to add a placeholder to select for example: '--- Select ---'
You can modify your html as shown below.
<select>
<option value="" disabled selected>--- Select ---</option>
<option value="1">C#</option>
<option value="2">C++</option>
</select>