Hi @Dmitry Novik • Thank you for reaching out.
Unfortunately, as of now it is NOT possible to set the Custom page URI for B2C user flow using the Graph API. All the operations that are currently supported in Azure AD B2C via Graph API are documented here: Manage Azure AD B2C with Microsoft Graph.
You may consider posting feedback regarding this at Azure Feedback Portal, which is monitored by Microsoft product teams for feature enhancements.
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