To create the extension attribute extension_mustResetPassword, you should upload the policy and create one account.

[Jimish-Shah24]

When you say "Upload the policy and create one account", do you mean to say, I need to create account through sign up of this custom policy?

Policy: https://github.com/azure-ad-b2c/samples/tree/master/policies/force-password-reset-first-logon

[JasSuri]

Yes sign up an account using the policy in the sample

https://github.com/azure-ad-b2c/samples/blob/master/policies/force-password-reset-first-logon/policy/TrustFrameworkExtensions.xml#L57