How to add mark (like € or %) into a text field ?
I see 3 possibilities:
Use a custom form validator that will check that the mark is still present and provide an error message indicating that the mark is mandatory
Use another non editable field just beside the editable text field. It will contain only the mark. You will need to modify css in order to have it always stuck to the text field
Use a custom HTML code by using the HTML widget