Hello Prateek Rathore,
Thank you for posting your query in the Microsoft Q&A Community.
I understand that you would like to set up a password reset custom policy using a username.
Kindly use this sample link below to achieve this.
https://github.com/azure-ad-b2c/samples/tree/master/policies/password-reset-with-Username-SMSVerify
However, you need to make some adjustments as the verification process here is via SMS and not email.
Let me know if further assistance is needed,
Babafemi