Prompts are useful for situations where users are expected to enter data or make a selection. You can write a message to prompt users to enter a value into a date/time field, numeric field, text field, password field, or drop-down list, or prompt users to select an option from a drop-down list, list box, or radio button group.
Note:
If users do not enter a value into the field and try to submit the form, the error message field is required appears. However, users can save and close a PDF form without providing recommended or required values. In this case, no messages appear to prompt users for input.
3.
In the Empty Message box, type the prompt. If applicable, the prompt should specify the required input format. For example, if you defined an edit pattern, the user input must conform to the edit pattern.
See also 

To prompt users to enter data