Set page template programaticlly in plugin

You can use the page_template filter to load a page template from the plugin directory:

function wpd_plugin_page_template( $page_template ){
    if ( is_page( 'namnam-login' ) ) {
        $page_template = dirname( __FILE__ ) . '/namnam-login.php';
    }
    return $page_template;
}
add_filter( 'page_template', 'wpd_plugin_page_template' );