trigger setGuid on dftly_app_contact__c (before insert) {
  For (dftly_app_contact__c dac: Trigger.new){
    Blob b = Crypto.GenerateAESKey(128);
    String h = EncodingUtil.ConvertTohex(b);
    String guid = h.SubString(0,8)+ '-' +
                  h.SubString(8,12) + '-' +
                  h.SubString(12,16) + '-' +
                  h.SubString(16,20) + '-' +
                  h.substring(20);
    system.debug(guid);
    dac.Guid_App_Contact__c = guid;
  }
}