How to use syncPurchases() in React Native

I migrated my app to RC, however my previous users are having the sync problem.

Previously, I was not using a back-end for the subscriptions, so I was using the client-side receipt validation on iOS (React Native IAP package).

I am trying to sync my previous users, but a little bit confused about the steps;

if (isSubscribedInOldSystem && !isSubscribedInRevenueCat) {

Purchases.syncPurchases();

}

1 - What should I provide to the syncPurchases() function?

2 - If I need to provide the receipt, should I provide full receipt? or just the receipt body? I mean, I was using this validation method;

const receiptBody = {
      'receipt-data': receipt, // Should I provide this to RC?
      password: 'PASSWORD',
    };

 await IAP.validateReceiptIos(receiptBody, false) 
      .then(response => {
        try {
          const renewalHistory = response.latest_receipt_info;
          const expirationDate = renewalHistory[0].expires_date_ms;
          const isExpired = Date.now() > expirationDate;

          if (!isExpired) {
            setIsUserHasReceiptOLD(true);
          }
        } catch (error) {
        }
      })
      .catch(e => {
      });

Thank you in advance,

Best,

Uğur