我不确定您是否已将此函数连接到适当的操作。此外，您还没有正确连接字符串：

而不是$newdestinationurl = $url/$destinationurlslug ; 应该是这样的$newdestinationurl = $url."/".$destinationurlslug ;

完整代码请参见以下内容：

function ninja_forms_handler() {
  add_action ( \'ninja_forms_post_process\', \'change_ninja_forms_landing_page\', 1, 2 );
}
add_action(\'init\', \'ninja_forms_handler\');

function change_ninja_forms_landing_page(){
    global $ninja_forms_processing; 

    $form_id = $ninja_forms_processing->get_form_ID(); 

        if( $form_id == 1 ){ 
        $destinationurlslug = $ninja_forms_processing->get_field_value( 1 );         
        $url = "http://www.blablabla.com";
        $newdestinationurl = $url."/".$destinationurlslug ;        
        /* $url now contains our new redirection page. Let\'s update the form setting. */
        $ninja_forms_processing->update_form_setting( \'landing_page\', $newdestinationurl ); 
    }     
}