How can I pass Unique ID to Infusionsoft Custom Field?

I have the Unique ID widget on my form. I would like that data passed to the contact's custom field in Infusionsoft. How can I do that?

You have already mapped the Unique ID widget to your Infusionsoft field upon checking. I presumed you have sorted this out already or are you having issues with it? 

This method worked for me when testing, so if the Infusionsoft fields support it, the widget should pass the data.

The Referral Code in my screenshot is a textbox field type.