a.js

export const Page = (props) => {
  const [foo, setFoo] = useState('foo')
  
  useForm({
    //...
    loadInitialValues: async () => { 
      setFoo('bar')
      console.log(foo) // 'bar'
    }
    onSubmit: async (values) => {
       console.log(foo) // 'foo'
    }
  })
}