I'm just adding more info here :)
As Declan said, you need to use CreatePrompt etc... and after you have an instance of the dialog, you can just do that:
yourPromptDialog.firstInputField.customTextValidator = new EmptyTextValidator();
yourPromptDialog.secondInputField.customTextValidator = new EmptyTextValidator();
Of course you replace the "new EmptyTextValidator();" with another validator. You can see the tutorial on input fields here: https://www.youtube.com/watch?v=DuRcl3JuH0U
~ Yohan