使用if语句重新提示用户输入数据

I'm trying to prompt for a first/last name in this code, but I cannot figure out how to re-prompt a user for the name if they forget to fill out the field. Right now if my code does not get any input and the user clicks onwards, I have it so a message box will pop up and then the program will close. I want to be able to just re-prompt for the exact same step without it getting all out of order.I'm trying to prompt for a first/last name in t