Using a Custom Taxonomy to Set Post Type

Hello Rebecca,

I understand that you want to set the post type based on the select field after submitting the frontend submission form. I share with you an example of how to make it work:

1. I have a field group and a simple select field with the options is the CPT slug and label:

[
    'name'              => __( 'Content type', 'your-text-domain' ),
    'id'                => 'content_type',
    'type'              => 'select',
    'options'           => [
        'book' => __( 'Book', 'your-text-domain' ),
        'post' => __( 'Post', 'your-text-domain' ),
        'page' => __( 'Page', 'your-text-domain' ),
    ],
],

2. This field group is added to the frontend form.

3. I use this code to update the post type of the submitted post based on the select:

add_action( 'rwmb_frontend_after_process', function( $config, $post_id ) {
    if ( isset( $_POST['content_type'] ) ) {
        $post_type = $_POST['content_type'];

        $my_post = [
            'ID' => $post_id,
            'post_type' => $post_type

        ];

        wp_update_post( $my_post );
    }
}, 10, 2 );

Refer to the documentation
https://docs.metabox.io/extensions/mb-frontend-submission/#form-hooks
https://developer.wordpress.org/reference/functions/wp_update_post/

Note: this is an example. If it doesn't work or does not meet your requirements, you can contact us here to request a customization service https://metabox.io/contact/