Let’s pre-select audi if the current URI is demo/cars and the facet isn’t already in use:

add_filter( 'facetwp_preload_url_vars', function( $url_vars ) {
    if ( 'demo/cars' == FWP()->helper->get_uri() ) {
        if ( empty( $url_vars['make'] ) ) {
            $url_vars['make'] = [ 'audi' ];
        }
    }
    return $url_vars;
} );

When a user visits http://yoursite.com/demo/cars/, Audi will be pre-selected.

Setting the correct value

Depending on the facet type, the value may need to be a multi-element array. Some examples:

// date range
$url_vars['your_date_field'] = [ '2018-01-01', '2018-12-31' ];

// number range
$url_vars['your_number_field'] = [ '25', '500' ];

// proximity (lat, long, radius, label)
$url_vars['your_proximity_field'] = [ '38.9072', '-77.0369', '10', 'Washington%2C%20DC' ];

Pre-select a facet choice after a reset

If you are using a Reset facet, the pre-selected option will be deselected again after a reset.

The following code re-selects that option again when a user clicks the Reset button/link:

<?php
add_action( 'wp_footer', function() {
?>
<script>
document.addEventListener('facetwp-loaded', function() {
    if (! FWP.loaded) {
        FWP.hooks.addAction('facetwp/reset', function() {
            FWP.facets['make'] = ['audi'];
        });
    }
});
</script>
<?php
}, 100 );

See also