When using iframes for Custom Menu Links inside of Go High Level - you may see a Refused to Connect message that looks like this:
The site you are iframing in has to allow itself to be added inside of the iframe. If you are seeing this error message it means that the site has explicitly denied access.
You can quickly test to see if a site or page is able to be iframed by using a site like:
What Determines if a Page is able to be IFramed
When a server sends a page to the browser, it adds headers. These include data like cookies, the type of file being sent down as well as if that page can be iframed or not. Headers are set on the server side so if you are dealing with a PHP file, it would be set on the PHP side.
If you are embedding an app like Google Docs or Google Drive, search for help docs that talk about enabling embedding.
Most payment apps like Stripe will not allow themselves to be embedded.
A good number of apps also don't allow themselves to be embedded as well.
If you do not have access to the app itself, the only real solution is to link to the page as a new window.
In the Customizer, it would look like this:
If you are using Go High Level Custom Menu Links it would look like this: