How to get Offer start date in Fast formula to Default input values

Summary:

Creating FF to default input value for payroll element attached to Individual comp plan.

In FF want to fetch offer start date from when & why section & also another input value which is date field.

If both the date are not null then default the 3rd input value.

not able to fetch these date items in FF.

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INPUTS ARE
1st_Cash_Payment_Date,
Start_date

If 1st_Cash_Payment_Date > Start_date