Usins Default Values in a Dialog Prompt and Response

I am trying to create a Dialog that updates a custom currency field called PO Value with a number.  I have created a Prompt and Response to enter the data and am trying to make it display the current value of the field in the box so if it has not changed the user can just click Next.

However although I have put PO Value as the Default Value (in the Response Details) it does not use it.  If I just type a number in there it does.

Can anybody help please.

Prompt and Response

Dialog Screen