Hi,
you can use this code

var lightbox = require("elgg/lightbox");
var spinner = require("elgg/spinner");

if(verified === 1) {
  lightbox.open({
    photo: false,
    close: elgg.echo("next"),
    title: 'your title',
    href: "link/to/form-you-want-to-load/for-example",
    onClosed: otherFunctionIfYouNeed()
  }); }

Thank you very much!

I was also wondering, which is the best way to define the link to the form i want to load? I mean how can I safely define a valid link to myy form?

It means form page, you should to create it in the elgg style

@Ramon Learn docs. Search on community.

You can find some answers on this site also.