auth.js

const handler = new DbAuthHandler({
  ...

  signup: {
    handler: async ({ username, password, userAttributes: { birthDate } }) => {
      //
      validate(birthDate, ...)

      // --

      try {
        await db.user.create({ data: { ... } })
      } catch (err) {
        ...
      }
    }
  }
})

Signup.jsx

const SignupPage = () => {  
  const { signUp } = useAuth()
  
  const onSignup = () => {
    ...
    
    const res = await signUp({ username, password, birthDate })
  }
  
  return ... 
}