Text field custom format only when field is completed

I am creating a form that I need a text field to have the format of "Model: " & text field value and then when it is blank for it to be just blank.

I put in event.value = "Model: " + event.value; in the custom format of the properties of the text field it set to format correctly, but if there was nothing inputted into that field it still says Model: so what do I need to do to make this only happen when there is something inputted into the field.