In case you have implemented Identity Management and see a blank Double Opt-in confirmation screen after clicking on the verification link received in your email (e.g. https://piano.io/?verification_code=DOVCFQK1ZWIZ62), you will need to add your website's URL to the list of Authorized JavaScript Origins in the Identity Management configuration under Edit Business.
Please note that depending on your setup, you might need to add your website's URL with or without a forward slash character / at the end.