Hi @Michelle Sollicito ,
There isn't an in-built capability of using a banned password list in B2C and it is not currently on the roadmap.
You would have to use custom policies for this and can follow the sample here: https://github.com/azure-ad-b2c/samples/tree/master/policies/banned-password-list-no-API
If you would like to leave feedback to have this capability added out of the box, you can do so in the feedback forum which the product team routinely monitors: https://feedback.azure.com/
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