Need help: Open API 3.0 generator

Can anyone help me with the following Open API 3.0 confusion.

Say I want to specify an endpoint for a sign-up use case.

POST /signup

The endpoint (on the frontend side) has a requestBody with a JSON:

{
"email": "user@user.com",
"password": "blaa",
"passwordConfirm": "blaa"
}

The endpoint (on the backend side) should look like this:

{
"email": "user@user.com",
"password": "blaa",
"passwordConfirm": "blaa",
"handle": "username",
"createdAt": "2021-05-28T12:39:47.802Z"
}

So there should be two more values, that only are handled by the backend side (createdAt and handle being initialized by the backend).

My goal is to generate the frontend and backend which should output different code:

• frontend: email, password, passwordConfirm
• backend: email, password, passwordConfirm, handle, createdAt

Is this something I can specify with Open API 3.0?
How would the specification look like?

Thanks for your help.