useMutation.js

const AddUser = useMutation({
      mutationFn: ( user ) => {
        return fetch('https://jsonplaceholder.typicode.com/users',
        {
          method:'post',
          headers: {
             "Content-Type": "application/json",          
          },
          body:JSON.stringify( user )
        }).then( res =>  res.json() )
      })