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SObjectType contactType = Schema.getGlobalDescribe().get('Contact');


 Map<String, Object> requestBody = (Map<String, Object>) JSON.deserializeUntyped(RestContext.request.requestBody.toString());

    SObject newContact = contactType.newSObject();

  
    newContact.put('FirstName', 'John'); // Change this value as needed
    newContact.put('LastName', 'Doe'); // Change this value as needed
    newContact.put('AccountId', acc.Id); // Associate with the current Account