New answers tagged embed
Not sure why Roland's comment is marked down because using Jetpack with it's "Shortcode embeds" setting turned on makes the embed code work. I have just been using a theme with the latest version of WP 4 and the SoundCloud shortcode only worked once I activated Jetpack Shortcode Embeds.
The code you use is incomplete, but otherwise I'm not sure what you mean by WP "curtailing." Assuming you (or someone) with admin or editor permissions is able to use the post editor, you should be able to add "unfiltered html" via the Text screen. If you (or someone) needs to enable users with lower permissions to do so, you would need to override.
I don't think sending your user off site is the best handling in the first place. I would honestly just use Gravity Forms and use the User Registration Add on - if you are already paying for your plugins. It just seems to weird to me to send your user offsite to register, but I'm probably missing data and it doesn't look like wufoo has User Registration ...
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