use SilverStripe\ORM\DataObject;
use SilverStripe\CMS\Model\SiteTree;
use Moo\HasOneSelector\Form\Field;

class Resource extends DataObject
{
    //...
}

class Page extends SiteTree
{
    //...
    private static $has_one = [
        'Resource' => Resource::class,
    ];

    public function getCMSFields()
    {
        $fields = parent::getCMSFields();

        $resource = Field::create('Resource', 'Resource', $this, Resource::class);
        $fields->addFieldToTab('Root.Main', $resource);

        return $fields;
    }

    //...
}