function render_posttitle( $form_id, $post_id, $form_settings ) {
    $value = get_post_field( $post_id, 'post_title', true );
    if ( $post_id ) {
        $value = get_post_meta( $post_id, 'meta_posttitle', true );
    }
    ?>
    <div class="wpuf-label">
        <label>A demo field</label>
    </div>
    <div class="wpuf-fields">
        <input type="text" name="my_custom_field" value="<?php echo esc_attr( $value ); ?>">
    </div>
    <?php
}
add_action( 'hook_posttitle', 'render_posttitle', 10, 3 );

/**
 * Update the custom field when the form submits
 *
 * @param type $post_id
 */
function update_hook_posttitle( $post_id ) {
    if ( isset( $_POST['my_custom_field'] ) ) {
        update_post_meta( $post_id, 'meta_posttitle', $_POST['my_custom_field'] );
    }
}

add_action( 'wpuf_add_post_after_insert', 'update_hook_posttitle' );
add_action( 'wpuf_edit_post_after_update', 'update_hook_posttitle' );