Do you need it on home/front page only? If so, try this:
add_action( 'template_redirect', function() {
if ( is_front_page() && !is_user_logged_in() ) {
wp_redirect( home_url( '/presentation' ) );
die();
}
} );
If you want ALL pages to WordPress’s default login page
add_action( 'template_redirect', function() {
if ( !is_user_logged_in() ) {
auth_redirect();
}
} );