Creating multiple resources in tandem with JSONAPI:Resources for Rails

Question

On my application, I would like to add a post and user in tandem using jsonapi:resources. A user can be created already. And a post can be created already if someone is signed in. However for this special scenario I would like a user to be able to submit their first post while creating their account. Here is the payload that would be submitted.

{
"data":{
    "user":{
        "attributes":{
            "name": "blah blah",
            "age": 20,
            "address": "mickey mouse street"
        }
    },
    "relationships": {
        "post": {
            "content": "this is a new post",
            "type": "blog"
        }
    }
}

}

Is this at all possible? If so in which resource file would I create them both? Would it be the resource file for posts or for users? Or would it be best practice to create the user first and then once the user has been created make a separate call to create the post? Thanks!