Add to confirmation text for unpaid events in green box

Hi, Can you please tell me how to add a line of text to the green box area that displays when someone RSVPs to an unpaid/free event?

Right now it says "Excellent! We've got you marked as coming and we'll see you there!"

I just want it to say "Excellent! We've got you marked as coming and we'll see you there! Be sure to purchase your tickets below..."

Thank you.