The Sign-in and Audit APIs aren't supported in Azure AD B2C yet. You can request this feature in the Azure AD B2C feedback forum.
The Azure AD B2C FAQ states:
What reporting and auditing features does Azure AD B2C provide? Are they the same as in Azure AD Premium?
No, Azure AD B2C does not support the same set of reports as Azure AD Premium. However there are many commonalities:
- The sign-in reports provide a record of each sign-in with reduced details.
- Audit reports are available in the Azure portal, under Azure Active Directory> ACTIVITY-Audit logs>Choose B2C and apply filters as desired. Both admin activity as well as application activity are covered.
- A usage report, covering number of users, number of logins, and volume of MFA is available at Usage Reporting API
Although not super clear, it states that sign-in and audit reports are available in the portal. The only API available is for Usage Reporting.
That being said, you can enable the sign-in and audit reports, but you'd need to get Azure AD Premium for that. However, as indicated above, it'll won't have the same level of detail as in regular Azure AD.