functions.php

<?php
/**
 * WPForms encoded query string
 * @author Bill Erickson 
 * @see https://www.billerickson.net/code/wpforms-encoded-query-string/
 *
 */
function be_wpforms_encoded_query_string( $field_id, $field_submit, $form_data ) {

    if ( empty( $field_submit ) || 'encoded' !== $form_data['fields'][ $field_id ]['css'] ) {
        return;
    }

    wpforms()->process->fields[ $field_id ]['value'] = sanitize_text_field( base64_decode( $field_submit ) );
}
add_action( 'wpforms_process_format_hidden', 'be_wpforms_encoded_query_string', 20, 3 );