Currency field with message if left unselected

Hello,
I'd like to ask how can I make the "Currency" field in the Donation Information section of my form, to be "required".
Because now, even if someone doesn't select any currency, he can still procceed with the payment.
I'd like to be a "required" field.
Or even better, if the donor leave this field without selection, a message will display by saying something like "...please select a currency..."

Thank you,

Apos