How can I display a custom form in a popup?
You can do it with the Lightbox module, for example using the following.

&lt;a href="calc.html" rel='lightframe'&gt;&lt;img src="sites/all/images/calc.png" /&gt;&lt;/a&gt;


Don't forget to add rel='lightframe'. See this page at the bottom, and click on "Mortage calculator."

The other option is the Popup Filter module which allows builders to popup tooltip-like text, nodes, blocks, menus, forms, views and PHP-generated content. Even you can mention the block to appear in popup window. And it has the option to popup the login form, register form etc.

Also look at the Popups API (Ajax Dialogs) module.


  API for building modal ajax dialogs. Its focus is on putting forms inside of ajax popup dialogs.
  Current example can present pages as popups (of the lightbox type). This is amazingly useful for things like confirmation pages. And the best part is it can be applied retroactively to a page without needing to modify the generating module, and (of course) it degrades gracefully.