Limit SearchWP Native/Default results to Category that has 'foobar' slug

searchwp-customizations.php

<?php

// Limit SearchWP Native/Default results to Category that has 'foobar' slug.
add_filter( 'searchwp\native\args', function( $args, $query ) {
    if ( ! isset( $args['tax_query'] ) || ! is_array( $args['tax_query'] ) ) {
        $args['tax_query'] = [];
    }

    $args['tax_query'][] = [
        'taxonomy' => 'category',
        'field'    => 'slug',
        'terms'    => 'foobar',
    ];

    return $args;
}, 20, 2 );