backend/config/api/forms/exports.config.js

module.exports = exports = {
    subtree: {
        fields: {
            PageId: {
                required: true,
                coerce: String,
            },
            format: {
                required: true,
                coerce: String,
                in_set: ['html', 'pdf', 'markdown', 'json'],
            }
        }
    }
}