How to force load a page in plugin?

If /foo doesn’t exist as a page, you can map the URI to another page with a rewrite rule:

function wpa_rewrite(){
    add_rewrite_rule(
        'foo/?$',
        'index.php?page_id=3',
        'top'
    );
}
add_action( 'init', 'wpa_rewrite' );