No, never. When paying for shares OnMarket members have two payment options - BPAY and Payment Express.
Your online banking credentials are not required to pay via BPAY.
If paying by Payment Express, you will be asked for your online banking credentials by Payment Express, which is a third party payment platform. The sign in process is hosted on Payment Express servers and is external to the OnMarket site and app. For more information refer to the payment express security guide.