Hi @Valleix • Thank you for reaching out.
This is not a case specifically for Azure AD B2C or Single Page Applications. Underscore is not a supported character in reply url for the application with "signInAudience" set to "AzureADandPersonalMicrosoftAccount" or "PersonalMicrosoftAccount". Basically the applications that support Microsoft Accounts (previously known as Live IDs).
- In Azure AD B2C tenant, the signinAudience is set to AzureADandPersonalMicrosoftAccount, if you select the Accounts in any identity provider or organizational directory (for authenticating users with user flows) option.
- In standard Azure AD tenant, the signinAudience is set to AzureADandPersonalMicrosoftAccount, if you select the Accounts in any organizational directory (Any Azure AD directory - Multitenant) and the signinAudience is set to PersonalMicrosoftAccount if you select the personal Microsoft accounts (e.g. Skype, Xbox) option during app registration.
As of now, there is no workaround available for these applications to use reply URLs with an underscore. You may post feedback regarding this at the User Voice portal which is monitored by the product team for feature enhancements.
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