What's chat plugin?

If you mean this plugin, you can make it compatible with Elgg 4 using documentation

So I have a mandatory form I want them to fill out before the access any other page on the site.

1 - In the form:

Add 'required' => true param to the mandatory field.

2 - In the action:

Check the value for the mandatory field:

if(!$request->getParam('param')) {
      return elgg_redirect_response($url);
}

3 - Validate the field:

You may want to validate all values in the form before sending them to server (via action).

You can use some libs for this (google it).

Look at this plugin also.

Sorry, I'm looking to not give them access to any other pages until they feel out a form.

Usual method:

http://learn.elgg.org/en/stable/guides/javascript.html#id3

Some examples:

https://github.com/Elgg/Elgg/blob/4.x/engine/lib/views.php#L1384

Use https://github.com/ColdTrick/oembed instead

Thank you Nikolai! I shall try it out. I hope it also renders in river, discussion/blog replies etc. of URLs.

Thanks!