By default, the Prompt property is empty. The Prompt gets displayed only if the control's Style property is DropDown or Simple ( as an editable area is provided in the control's label ). A prompt is a label or short instruction placed inside an editable drop-down list as its default value. Unlike static text, prompts disappear from the screen once users type something into the combo box or it gets input focus. Use the SingleEdit property to specify whether the control's label displays a single or multiple columns. The EditText property to change the text being displayed in the control's label, while the Style property is DropDown or Simple. 

Use a prompt when:

• The prompt is primarily for identifying the purpose of the list in a compact way. It must not be crucial information that users need to see while using the combo box.
• For example, prompts like Select an option or Enter a filename and then click Send are unnecessary.
• The prompt text must not be confused with real text, so you can use the <fgcolor> HTML tag to specify a different foreground color.

For instance, the following Prompt "<font Arial;8><fgcolor=808080><i><b>type to search<r><img>1</img>" is showing as follows , and so the image gets aligned to the right due <r> field, and the text gets aligned to the left.

If you are changing the column's prompt at runtime, you need to call the EditText property as follows, else the prompt will not be updated until a change occurs:

With ComboBox1
        .Columns(0).Prompt = "<img>1</img><i><fgcolor=808080>type to search</fgcolor></i>"
        .EditText(0) = ""
End With

The following screen shot shows how the prompt is disappearing as soon as the user starts typing characters, and a new item gets selected:

 

The Prompt property supports the built-in HTML tags listed here.