When you subscribe to the paid plan, the create your own form button will be automatically removed from the confirmation message. Open your form in Google Forms > click on the add icon > click Formfillable > click More options > click Customize > Edit page will open in a new tab > In the Edit page, click on the gear icon next to the Submit button > click On submit > select Show customized message for this button > enter the message and click Save. Please try this and confirm.