Default Effective Start Date to Pay Period Begin Date using Auto complete rule

Summary:

The business requirement is to default the Effective start date for the below transactions to current Pay Period Begin date. I am trying to derive the Pay Period Begin date for the current pay period but would need help to do it in auto complete rule. As advised, I am using "Business Object Defaulting Rule" on "When and Why" object. Use case 24 is approved for Guidewell Mutual project against this requirement.

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Need help with code snipped that can be used to fetch the Pay Period Begin date to default it for the Effective Start Date