How to change login validation rule?

I am using Okta restful API “{{url}}/api/v1/users?activate=true” to create a user account. I need to fill in some required parameters like login, firstName, lastName, email etc. The “login” is used as Primary Authentication username. But it requires email format. If not, I will get this error response: “login: User name must be in the form of an email address (e.g.”, How can I change the login validation rule to be a general username not an email address. There is an email attribute on the registration, so no point to keep another email format attribute.

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