What exactly do you mean by login pop up? Do you mean this: ?
If so then it would depend on the type of script. The default login pop up that you can enable on your forum to guests (shown in above screenshot) supports html and thats it (I believe anyway). So if the script is an html script, then yes you can. If it is like a javascript or css code, then no there is no way to directly add it into the login pop up.
I am sure there might be other ways to add custom scripts by making ones own login pop up from scratch using code. I am not 100% sure though whether or not this is actually possible using custom codes/scripts.
What I can tell you with about 97% certainity is that the default login pop up option DOES support html and only html.
@brandon_g, you are right, by default, the popup only supports HTML, but I think it can be edited to support css and javascript. If anyone needs this, I can try and make a code for that.
Posts : 80 Reputation : 7 Language : US English / Dutch Location : 35 miles NWW of Los Angeles Airport (LAX) when able, currently living 1 mile SWW of Amsterdam Airport (AMS).
Had a quick glance at this using the dev tools. And yea's indeed, they're making use of scripts 'round here. Buttttt... again... they've been copyrighted. So, as much as I'd like to be of help, from where I'm sitting at, you'll first be needing to take 'em bumps... Srry.