Pre-fill the search box if not filled

functions.php

<?php

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

add_filter( 'facetwp_render_params', function( $params ) {
    foreach ( $params['facets'] as $key => $facet ) {
        if ( 'keywords' == $facet['facet_name'] ) { // change "keywords" to your actual facet name
            if ( empty( $facet['selected_values'] ) ) {
                $params['facets'][ $key ]['selected_values'] = 'circ';
            }
        }
    }
    return $params;
} );