How can I set up multiple callback domains in a sandbox environment?

Please enter your preferred domains separated by commas in the AllowedAuthRedirectDomains setting in OpenAPI Configurations on PayPay for Developers and save the settings. When you save the setting, please remember to push the Save Changes button at the bottom after selecting the green check mark next to the input field.

Note: For the Web Payment, you can use redirectURL without registering callback domains.