As reported by cliffmeyers:

I've been working on the Github Creation API and have the first simplest flow wired up for creating an org folder. I am encountering a problem although I found the workaround for it. I wanted to run it by you to see what you think.

First the UI calls this endpoint to get the credential that I previously created:
http://localhost:8080/jenkins/blue/rest/organizations/jenkins/scm/github/

It returns an object like this (omitting _class and _links)

{
    "credentialId": "github",
    "id": "github",
    "uri": "https://api.github.com"
}

Then I make a call to fetch the orgs which works fine:
http://localhost:8080/jenkins/blue/rest/organizations/jenkins/scm/github/organizations/?credentialId=github

Then when I make the creation call I send an object like this:

{
    "name": "cliffmeyers",
    "$class": "io.jenkins.blueocean.blueocean_github_pipeline.GithubPipelineCreateRequest",
    "scmConfig": {
        "uri": "https://api.github.com",
        "config": {
            "orgName": "cliffmeyers",
            "repos": []
        }
    }
}

This works fine. However if I add the credentialId: 'github' to this it fails saying invalid credential.