FacetWP – add the “URL prefix” setting temporarily

<?php

// Add to your (child) theme's functions.php

add_filter( 'facetwp_settings_admin', function( $settings, $class ) {
    $settings['general']['fields']['prefix'] = [
        'label' => __( 'URL prefix', 'fwp' ),
        'html' => $class->get_field_html( 'prefix', 'dropdown', [
            'choices' => [ 'fwp_' => 'fwp_', '_' => '_' ]
        ] )
    ];
    return $settings;
}, 10, 2 );